System Information intel core i3-8100 CPU 3.60 Ghz 16G RAM
Operating system: Windows 10 64bit
Graphics card: NVIDIA Quadro P620

Blender Version
Broken: 2.91
Caused by rB4212b6528afb: Image Editor: Enable New Drawing

Short description of error
Joining two objects with different UV's destroys the UV's of one or more objects. It seems to match an earlier bug from 2014 #T37182 which is discussed here https://developer.blender.org/T37182. The objects whose UV's are destroyed have all their Vertices stacked on top of each other at the bottom left corner. I I upgraded from a previous version (think it was 2.90a), but it is still happening. It wasn't happening on V2.82 which was the last one I used.

Exact steps for others to reproduce the error
I made 3 parts of a piece of furniture, UV mapped them, assigned them their material properties and joined them. I managed to join 2 pieces successfully, of which one was a simple plane with 4 vertices. It only worked after assigning the material properties, so I did the same with the last piece, but to no avail. No matter which way I joined them.

This is in fact an issue drawing caused by rB4212b6528afb07d9b2962566ae9c4251282d387f, (the join is working).
Notice face-dots are drawing properly when face-mode is selected.
It looks to be an issue caused by using the render layer instead of the active layer for drawing UV's.