John Kitchin <jkitchin@andrew.cmu.edu> writes:

> However, if I edebug `calendar-make-temp-face' and step through each line,
> then it works the same as using a face. And, after that it seems to work
> most of the time.

It turned out that the problem is caused by interaction between Org
mode's overlay and calendar.el's overlay. They have the same priority,
so the order they are applied depends on subtle details of the code and
on when redisplay occurs.
That's why edebug yielded different results.

See https://debbugs.gnu.org/cgi/bugreport.cgi?bug=69271

I fixed this subtle issue on main.
https://git.savannah.gnu.org/cgit/emacs/org-mode.git/commit/?id=67d937248

--
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>