During my rebuild I did a distclean, wiped the old install directory and also cleared the eln-cache just to make sure everything was pristine. The function in question is reported as natively compiled:

c-determine-limit-no-macro is a native compiled Lisp function in
‘cc-engine.el’.

On Thu, 29 Apr 2021 at 14:12, Eli Zaretskii <eliz@gnu.org> wrote:

> Cc: 48097@debbugs.gnu.org
> Date: Thu, 29 Apr 2021 12:17:43 +0000
> From: Andrea Corallo via "Bug reports for GNU Emacs,
> the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>
> Alex Bennée <alex.bennee@linaro.org> writes:
>
> > I suspect this might be a duplicate of #48061 - re-building with the current master the bug has gone away and problems
> > with c-mode cause by strange code would certainly explain it.
>
> Hi Alex,
>
> okay shall we close this then?

Please don't close yet, I still see the problem.

It is strange that Alex says bug#48061 should solve this: the fix for
that bug changed comp-cstr.el, and I have that natively-compiled after
that change. Do I need to make sure other files are recompiled, for
example the CC mode files?