> Last question: does the file src/emacs-27.1/src/xdisp.c in the tree
> where you unpacked the snapshot have this line on or around line
> number 21811:
>
>   const int orig_face_id = it->face_id;
>

YES! That was it.
I made a major error during my build process and nothing was patched.
When I correctly patched my emacs the bug was gone!!!
The bug is fixed!

Thank you very, very much for everything!
Best wishes.