I'm seeing some problems when (attempting to) use checkpoints with multiple inferiors. I'll describe two such cases below...

Build gdb and make your CWD the gdb directory containing the gdb executable.  Then run the following test:

make check TESTS=gdb.multi/base.exp

This test should have all passes; I see 27 passes when I do it.  I'll use the programs associated with this test to demonstrate some bugs.

Now, start GDB as follows:

./gdb -q -iex 'set debuginfod enabled off'

Turning debuginfod off (or on) prevents GDB from asking the question about using debuginfod, allowing us to paste the following commands to GDB all at once.

Run the following commands within the GDB session:

file testsuite/outputs/gdb.multi/base/hello
start
add-inferior -exec testsuite/outputs/gdb.multi/base/goodbye
inferior 2
start
checkpoint
step
inferior 1
detach

Upon execution of the detach command, an internal error will occur.

A different internal error may be caused by doing 'restart 0' instead of 'detach'.

Example session showing the bug (detach):

kev@f39-1:gdb$ ./gdb -q -iex 'set debuginfod enabled off'
(gdb) file testsuite/outputs/gdb.multi/base/hello
Reading symbols from testsuite/outputs/gdb.multi/base/hello...
(gdb) start
Temporary breakpoint 1 at 0x401199: file /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/hello.c, line 51.
Starting program: /mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/hello 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Temporary breakpoint 1, main ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/hello.c:51
51	  alarm (240);
(gdb) add-inferior -exec testsuite/outputs/gdb.multi/base/goodbye
[New inferior 2]
Added inferior 2 on connection 1 (native)
Reading symbols from testsuite/outputs/gdb.multi/base/goodbye...
(gdb) inferior 2
[Switching to inferior 2 [<null>] (/mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/goodbye)]
(gdb) start
Temporary breakpoint 2 at 0x401199: -qualified main. (2 locations)
Starting program: /mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/goodbye 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Thread 2.1 "goodbye" hit Temporary breakpoint 2.2, main ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/goodbye.c:61
61	  mailand();
(gdb) checkpoint
checkpoint 1: fork returned pid 239943.
(gdb) step
mailand ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/goodbye.c:46
46	  glob = 46;
(gdb) inferior 1
[Switching to inferior 1 [process 239939] (/mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/hello)]
[Switching to thread 1.1 (Thread 0x7ffff7cd3740 (LWP 239939))]
#0  main ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/hello.c:51
51	  alarm (240);
(gdb) detach
/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:624: internal-error: lwp_lwpid_htab_add_lwp: Assertion `slot != NULL && *slot == NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
----- Backtrace -----
0x4ef132 gdb_internal_backtrace_1
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/bt-utils.c:122
0x4ef132 _Z22gdb_internal_backtracev
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/bt-utils.c:168
0x8d9074 internal_vproblem
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/utils.c:396
0x8d92f8 _Z15internal_verrorPKciS0_P13__va_list_tag
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/utils.c:476
0xa1e431 _Z18internal_error_locPKciS0_z
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdbsupport/errors.cc:58
0x6c4a41 lwp_lwpid_htab_add_lwp
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:624
0x6c4a41 add_initial_lwp
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:792
0x6c7d58 add_lwp
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:806
0x6c7d58 _Z21linux_nat_switch_fork6ptid_t
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:887
0x6c28fe fork_load_infrun_state
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-fork.c:219
0x6c343b _Z17linux_fork_detachi
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-fork.c:381
0x6c8bd8 _ZN16linux_nat_target6detachEP8inferiori
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:1436
0x6d8652 _ZN16thread_db_target6detachEP8inferiori
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-thread-db.c:1385
0x85e943 _Z13target_detachP8inferiori
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/target.c:2526
0x680aed _Z14detach_commandPKci
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/infcmd.c:2863
0x5224ac _Z8cmd_funcP16cmd_list_elementPKci
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/cli/cli-decode.c:2735
0x86cd0a _Z15execute_commandPKci
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/top.c:575
0x600c4f _Z15command_handlerPKc
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:552
0x60204a _Z20command_line_handlerOSt10unique_ptrIcN3gdb13xfree_deleterIcEEE
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:788
0x60149b gdb_rl_callback_handler
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:259
0x91ef44 rl_callback_read_char
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/readline/readline/callback.c:290
0x6015bd gdb_rl_callback_read_char_wrapper_noexcept
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:195
0x60173c gdb_rl_callback_read_char_wrapper
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:234
0x8a763f stdin_event_handler
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/ui.c:155
0xa1ed35 gdb_wait_for_event
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdbsupport/event-loop.cc:716
0xa1f826 _Z16gdb_do_one_eventi
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdbsupport/event-loop.cc:217
0x6e92c9 start_event_loop
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:407
0x6e92c9 captured_command_loop
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:471
0x6ebba4 captured_main
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:1324
0x6ebba4 _Z8gdb_mainP18captured_main_args
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:1343
0x446ad4 main
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/gdb.c:39
---------------------
/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-nat.c:624: internal-error: lwp_lwpid_htab_add_lwp: Assertion `slot != NULL && *slot == NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.

Example session showing the bug (restart 0):

kev@f39-1:gdb$ ./gdb -q -iex 'set debuginfod enabled off'
(gdb) file testsuite/outputs/gdb.multi/base/hello
Reading symbols from testsuite/outputs/gdb.multi/base/hello...
(gdb) start
Temporary breakpoint 1 at 0x401199: file /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/hello.c, line 51.
Starting program: /mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/hello 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Temporary breakpoint 1, main ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/hello.c:51
51	  alarm (240);
(gdb) add-inferior -exec testsuite/outputs/gdb.multi/base/goodbye
[New inferior 2]
Added inferior 2 on connection 1 (native)
Reading symbols from testsuite/outputs/gdb.multi/base/goodbye...
(gdb) inferior 2
[Switching to inferior 2 [<null>] (/mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/goodbye)]
(gdb) start
Temporary breakpoint 2 at 0x401199: -qualified main. (2 locations)
Starting program: /mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/goodbye 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Thread 2.1 "goodbye" hit Temporary breakpoint 2.2, main ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/goodbye.c:61
61	  mailand();
(gdb) checkpoint
checkpoint 1: fork returned pid 240018.
(gdb) step
mailand ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/goodbye.c:46
46	  glob = 46;
(gdb) inferior 1
[Switching to inferior 1 [process 240014] (/mesquite2/sourceware-git/f39-master/bld/gdb/testsuite/outputs/gdb.multi/base/hello)]
[Switching to thread 1.1 (Thread 0x7ffff7cd3740 (LWP 240014))]
#0  main ()
    at /mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/testsuite/gdb.multi/hello.c:51
51	  alarm (240);
(gdb) restart 0
/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-fork.c:736: internal-error: linux_fork_context: Assertion `oldfp != NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
----- Backtrace -----
0x4ef132 gdb_internal_backtrace_1
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/bt-utils.c:122
0x4ef132 _Z22gdb_internal_backtracev
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/bt-utils.c:168
0x8d9074 internal_vproblem
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/utils.c:396
0x8d92f8 _Z15internal_verrorPKciS0_P13__va_list_tag
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/utils.c:476
0xa1e431 _Z18internal_error_locPKciS0_z
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdbsupport/errors.cc:58
0x6c3e50 linux_fork_context
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-fork.c:736
0x6c3e50 restart_command
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-fork.c:761
0x5224ac _Z8cmd_funcP16cmd_list_elementPKci
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/cli/cli-decode.c:2735
0x86cd0a _Z15execute_commandPKci
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/top.c:575
0x600c4f _Z15command_handlerPKc
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:552
0x60204a _Z20command_line_handlerOSt10unique_ptrIcN3gdb13xfree_deleterIcEEE
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:788
0x60149b gdb_rl_callback_handler
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:259
0x91ef44 rl_callback_read_char
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/readline/readline/callback.c:290
0x6015bd gdb_rl_callback_read_char_wrapper_noexcept
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:195
0x60173c gdb_rl_callback_read_char_wrapper
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/event-top.c:234
0x8a763f stdin_event_handler
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/ui.c:155
0xa1ed35 gdb_wait_for_event
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdbsupport/event-loop.cc:716
0xa1f826 _Z16gdb_do_one_eventi
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdbsupport/event-loop.cc:217
0x6e92c9 start_event_loop
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:407
0x6e92c9 captured_command_loop
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:471
0x6ebba4 captured_main
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:1324
0x6ebba4 _Z8gdb_mainP18captured_main_args
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/main.c:1343
0x446ad4 main
	/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/gdb.c:39
---------------------
/mesquite2/sourceware-git/f39-master/bld/../../worktree-master/gdb/linux-fork.c:736: internal-error: linux_fork_context: Assertion `oldfp != NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.