I.

Consider the following test-case:
...
$ cat -n stmts-on-line.c 
     1  int
     2  main (void)
     3  {
     4    int a = 4;
     5
     6    a = 5; a = 6; a = 7;
     7
     8    return 0;
     9  }
...

II.

Compiled with gcc 7.5.0 and debug info:
...
$ gcc -g stmts-on-line.c 
...

We can step through the instructions on line 6 using stepi:
...
$ gdb a.out
Reading symbols from a.out...
(gdb) start
Temporary breakpoint 1 at 0x40049b: file stmts-on-line.c, line 4.
Starting program: a.out 

Temporary breakpoint 1, main () at stmts-on-line.c:4
4         int a = 4;
(gdb) s
6         a = 5; a = 6; a = 7;
(gdb) si
0x00000000004004a9      6         a = 5; a = 6; a = 7;
(gdb) si
0x00000000004004b0      6         a = 5; a = 6; a = 7;
(gdb) si
8         return 0;
...
and can see progress through the line number and instruction addresses.

That works as follows.  The instructions are at addresses:
...
  40049b:       c7 45 fc 04 00 00 00    movl   $0x4,-0x4(%rbp)
  4004a2:       c7 45 fc 05 00 00 00    movl   $0x5,-0x4(%rbp)
  4004a9:       c7 45 fc 06 00 00 00    movl   $0x6,-0x4(%rbp)
  4004b0:       c7 45 fc 07 00 00 00    movl   $0x7,-0x4(%rbp)
  4004b7:       b8 00 00 00 00          mov    $0x0,%eax
...
and the line number info table looks like:
...
  [0x00000125]  Special opcode 62: advance Address by 4 to 0x40049b and Line by 1 to 4
  [0x00000126]  Special opcode 105: advance Address by 7 to 0x4004a2 and Line by 2 to 6
  [0x00000127]  Advance PC by constant 17 to 0x4004b3
  [0x00000128]  Special opcode 63: advance Address by 4 to 0x4004b7 and Line by 2 to 8
...

So, put in DWARF standard terms, there are "recommended breakpoint locations" at 0x40049b, 0x4004a2 and 0x4004b7.  When we're stopped at such a location, the line without address is shown.

When we're stopped at an instruction which is not such a location, the line with address is shown (4004a9, 4004b0).

III.

Now consider the same test-case, compiled with gcc 10.0.1 and debug info:
...
$ gcc-10 -g stmts-on-line.c 
...

Now when stepping through the instructions on line 6 using stepi:
...
$ gdb a.out
Reading symbols from a.out...
(gdb) start
Temporary breakpoint 1 at 0x400496: file stmts-on-line.c, line 4.
Starting program: /data/gdb_versions/devel/a.out 

Temporary breakpoint 1, main () at stmts-on-line.c:4
4         int a = 4;
(gdb) s
6         a = 5; a = 6; a = 7;
(gdb) si
6         a = 5; a = 6; a = 7;
(gdb) si
6         a = 5; a = 6; a = 7;
(gdb) si
8         return 0;
...
we don't see progress, we seem to be stuck at line 6.

In this case, the instructions are at addresses:
...
  400496:       c7 45 fc 04 00 00 00    movl   $0x4,-0x4(%rbp)
  40049d:       c7 45 fc 05 00 00 00    movl   $0x5,-0x4(%rbp)
  4004a4:       c7 45 fc 06 00 00 00    movl   $0x6,-0x4(%rbp)
  4004ab:       c7 45 fc 07 00 00 00    movl   $0x7,-0x4(%rbp)
  4004b2:       b8 00 00 00 00          mov    $0x0,%eax
...
and the line number info table looks like:
...
  [0x00000129]  Special opcode 62: advance Address by 4 to 0x400496 and Line by 1 to 4
  [0x0000012a]  Set column to 5
  [0x0000012c]  Special opcode 105: advance Address by 7 to 0x40049d and Line by 2 to 6
  [0x0000012d]  Set column to 12
  [0x0000012f]  Special opcode 103: advance Address by 7 to 0x4004a4 and Line by 0 to 6
  [0x00000130]  Set column to 19
  [0x00000132]  Special opcode 103: advance Address by 7 to 0x4004ab and Line by 0 to 6
  [0x00000133]  Set column to 10
  [0x00000135]  Special opcode 105: advance Address by 7 to 0x4004b2 and Line by ...

Now all the instructions are also breakpoint locations, which explains why we don't see addresses.

IV.

It would be good if gdb can somehow show progress here, perhaps through something like:
...
(gdb) s
6         a = 5; a = 6; a = 7;
(gdb) si
6         a = 5; a = 6; a = 7;
                 ^
(gdb) si
6         a = 5; a = 6; a = 7;
                        ^
(gdb) si
8         return 0;
...