Maybe I have something.  Could you please check?

Please read the following list from the bottom up, i.e. re_match... calls maybe_quit etc.

maybe_gc
Ffuncall
call2
signal_or_quit (eval.c:1741)
quit (eval.c:1697)
process_quit_flag (eval.c:1657)
probably_quit (eval.c:1864)
maybe_quit (lisp.h:3681)
re_match_2_internal (regexp-emacs.c:4691)

If this is true a GC can be triggered under very specific circumstances involving edebug, if the comment in signal_or_quit is right.

And I might have used edebug, I'm not 100% sure anymore.