Ok, I've got some valgrind output:

Program terminated with signal SIGABRT, Aborted.
#0  __restore_sigs (set=set@entry=0x1ffeffe910) at ./arch/x86_64/syscall_arch.h:40
40              return ret;
(gdb) backtrace
#0  __restore_sigs (set=set@entry=0x1ffeffe910) at ./arch/x86_64/syscall_arch.h:40
#1  0x00000000004cf1c4 in raise (sig=sig@entry=6) at src/signal/raise.c:11
#2  0x00000000004c96ea in abort () at src/exit/abort.c:11
#3  0x0000000000452f16 in catchsig (sig=<optimized out>) at main.c:1231
#4  catchsig (sig=<optimized out>) at main.c:1217
#5  <signal handler called>
#6  get_meta (p=p@entry=0x4a5af50 "\260\232\243\001") at src/malloc/mallocng/meta.h:141
#7  0x00000000004ca62c in __libc_free (p=0x4a5af50) at src/malloc/mallocng/free.c:105
#8  0x0000000000459d57 in r_unref (tmp=tmp@entry=0x48728c8) at node.c:492
#9  0x00000000004456e5 in unref (r=0x48728c8) at awk.h:1898
#10 purge_record () at field.c:360
#11 0x0000000000445f36 in set_record (
    buf=0x549bdae "SIZE\t1KB\t192758\t112698119\nSIZE\t4KB\t462090\t1122421206\nSIZE\t16KB\t355596\t3138948391\nSIZE\t64KB\t413028\t13558910172\nSIZE\t256KB\t1014012\t165385758947\nSIZE\t1MB\t2546508\t1346405498466\nSIZE\t10MB\t480844\t1602796961"..., cnt=25, fw=0x0) at field.c:269
#12 0x000000000044e765 in inrec (iop=<optimized out>, errcode=errcode@entry=0x1ffefffa50) at io.c:608
#13 0x00000000004414f8 in h_interpret (code=<optimized out>) at interpret.h:1289
#14 0x0000000000402631 in main (argc=<optimized out>, argv=<optimized out>) at main.c:516
(gdb) frame 8
#8  0x0000000000459d57 in r_unref (tmp=tmp@entry=0x48728c8) at node.c:492
492             mpfr_unset(tmp);
(gdb) print tmp
$1 = (NODE *) 0x48728c8
(gdb) print *tmp
$2 = {sub = {nodep = {l = {lptr = 0x100000002, li = 0x100000002, ll = 4294967298, lp = 0x100000002}, r = {rptr = 0x4a5af50, preg = {0x4a5af50, 0x0}, av = 0x4a5af50, bv = 0x4a5af50,
        uptr = 0x4a5af50, iptr = 0x4a5af50}, x = {extra = 0x0, aptr = 0x0, xl = 0}, name = 0x4e <error: Cannot access memory at address 0x4e>, reserved = 75884000, rn = 0x8, cnt = 0,
      reflags = 4294967295}, val = {nm = {fltnum = 2.1219957919534036e-314, mpnum = {{_mpfr_prec = 4294967298, _mpfr_sign = 77967184, _mpfr_exp = 0, _mpfr_d = 0x0}}, mpi = {{
            _mp_alloc = 2, _mp_size = 1, _mp_d = 0x4a5af50}}}, rndmode = 78, sp = 0x485e5e0 "27499184", slen = 8, sref = 0, idx = -1, wsp = 0x0, wslen = 0, typre = 0x0}},
  type = Node_val, flags = 2108}


==43165== Conditional jump or move depends on uninitialised value(s)
==43165==    at 0x4CA82E: enframe (meta.h:204)
==43165==    by 0x462EAB: xrealloc (xalloc.h:173)
==43165==    by 0x462EAB: xpalloc (dfa.c:850)
==43165==    by 0x462F52: copy.isra.10 (dfa.c:2043)
==43165==    by 0x464BC1: dfaanalyze (dfa.c:2490)
==43165==    by 0x4679F2: dfacomp (dfa.c:3527)
==43165==    by 0x45F596: make_regexp (re.c:230)
==43165==    by 0x413740: make_regnode (awkgram.y:5026)
==43165==    by 0x416C08: yyparse (awkgram.y:505)
==43165==    by 0x41B443: parse_program (awkgram.y:2587)
==43165==    by 0x4025AB: main (main.c:469)
==43165==  Uninitialised value was created by a stack allocation
==43165==    at 0x4760E3: re_compile_internal (regcomp.c:755)
==43165==
==43165== Conditional jump or move depends on uninitialised value(s)
==43165==    at 0x4CA82E: enframe (meta.h:204)
==43165==    by 0x4D837D: __libc_realloc (realloc.c:46)
==43165==    by 0x46B977: test_realloc (regex_internal.h:476)
==43165==    by 0x46B977: re_dfa_add_node (regex_internal.c:1438)
==43165==    by 0x46BA7A: duplicate_node (regcomp.c:1643)
==43165==    by 0x46BC59: duplicate_node_closure (regcomp.c:1566)
==43165==    by 0x46E3CF: calc_eclosure_iter.isra.30 (regcomp.c:1745)
==43165==    by 0x47660C: calc_eclosure (regcomp.c:1708)
==43165==    by 0x47660C: analyze (regcomp.c:1235)
==43165==    by 0x47660C: re_compile_internal (regcomp.c:821)
==43165==    by 0x476EB9: re_compile_pattern (regcomp.c:256)
==43165==    by 0x45F0FA: make_regexp (re.c:215)
==43165==    by 0x413765: make_regnode (awkgram.y:5031)
==43165==    by 0x416C08: yyparse (awkgram.y:505)
==43165==    by 0x41B443: parse_program (awkgram.y:2587)
==43165==  Uninitialised value was created by a stack allocation
==43165==    at 0x4760E3: re_compile_internal (regcomp.c:755)
==43165==
==43165== Conditional jump or move depends on uninitialised value(s)
==43165==    at 0x4CA82E: enframe (meta.h:204)
==43165==    by 0x462D44: xmalloc (xalloc.h:147)
==43165==    by 0x46770C: xnmalloc (xalloc.h:134)
==43165==    by 0x46770C: dfassbuild (dfa.c:3455)
==43165==    by 0x46770C: dfacomp (dfa.c:3522)
==43165==    by 0x45F596: make_regexp (re.c:230)
==43165==    by 0x413765: make_regnode (awkgram.y:5031)
==43165==    by 0x416C08: yyparse (awkgram.y:505)
==43165==    by 0x41B443: parse_program (awkgram.y:2587)
==43165==    by 0x4025AB: main (main.c:469)
==43165==  Uninitialised value was created by a stack allocation
==43165==    at 0x4760E3: re_compile_internal (regcomp.c:755)
==43165==

Since it looks like a gawk bug, not directly anything to do with musl, I'll see if I can submit a bug report with the gawk maintainers.

   Ivo Palli

From: Ivo Palli <Ivo.Palli@bytesnet.nl>
Sent: Friday, September 24, 2021 16:38
To: musl@lists.openwall.com <musl@lists.openwall.com>; Rich Felker <dalias@libc.org>
Subject: Re: [musl] Musl compiled gawk coredumps. How to find out where?
 
>Do you think you can share your script and input? That would help anyone
>trying to reproduce the bug if they can't find the issue in the code via
>inspection.

I'm afraid not. But I will put some effort into helping to find the bug.