Further information:
* This bug does does not present itself if I start a session under X. The problem appears to be Wayland-specific.
* I built 28.1 from scratch with "--with-x-toolkit=no" and the bug disappears. The menu is in the correct location, although it does not look the same as it did.
* I built 28.1 from scratch with "--with-x-toolkit=yes" and no other changes and the bug is back.
Thanks for looking into this!