Joffrey suspected this could be an issue with safari. What is really odd is that the inspector shows that the menu should be placed appropriately on the screen. The pixels and numbers are correct.
This is made evident on safari if you uncheck “fixed” and then turn it back on in the menu placement, the menu then is correctly positioned.
Since this sort of use-case for BG images has been in effect since prior to 2020, I wonder if this is recent, or if this never worked, and this is the first time we are hearing of it.