oh, sorry, hit RETURN to get rid of all the autocompletions.
The following program fails with

 $/tmp/zt
 Converting 4 <aäc>
 Fail <Invalid or incomplete multibyte or wide character>

whereas musl gives

 $./zt
 Converting 4 <aäc>
 GOT <a*c>

I.e., i would expect replacement to happen, but instead (size_t)-1 is returned.

#include <stdio.h> 
#include <string.h>
#include <iconv.h>
#include <errno.h>
int main(void){
   char inb[16], oub[16], *inbp, *oubp;
   iconv_t id;
   size_t inl, oul;

   memcpy(inbp = inb, "a\303\244c", sizeof("a\303\244c"));
   inl = sizeof("a\303\244c") -1;
   oul = sizeof oub;
   oubp = oub;

   if((id = iconv_open("ascii", "utf8")) == (iconv_t)-1)
     return 1;
   fprintf(stderr, "Converting %lu <%s>\n",(unsigned long)inl, inbp);
   if(iconv(id, &inbp, &inl, &oubp, &oul) == (size_t)-1){
      fprintf(stderr, "Fail <%s>\n", strerror(errno));
      return 2;
   }  
   fprintf(stderr, "GOT <%s>\n", oub);
   iconv_close(id);
   return 0;
}

Have you tried to convert using //TRANSLIT?

No.  (Why, the input is valid and complete, no error should occur thus?  But 
likely you are looking at the actual libc code...)

I am in fact anything but a fan of these // modifiers, because iconv(3) is already a blackbox and therefore programs have no chance to perform cleanup on character set names; in effect they anything found in kinds of charset="" parameters "as-is" when calling iconv_open() (sic!).
Protection against to be implemented one day, search for slash and terminate, or so.

iconv should instead offer interfaces for character set name normalization, offer access to the charsets actually used by the successfully iconv_open()ed object, replacement character configuration, etc. etc., all via an explicit function.  But it cannot be helped.
A nice Sunday i wish.

With

   setlocale( LC_ALL, "");

and

   if((id = iconv_open("ascii//TRANSLIT", "utf8")) == (iconv_t)-1)

transliteration occurs, as expected.

(Note that transliteration is locale-dependent, so you need to configure a locale.)