I'm using XnView MP Linux, Version 0.93.1 64bits (Mar 7 2019) on Manjaro. Ratings and color markers for my photos are not saved. I tried rating in full screen mode via hotkey as well as in browser mode via the context menu. Once I exit XnView and re-open, they are gone.
When I switch to saving ratings and color markers in XMP files, everything is fine.
When I run XnView MP from the terminal, no errors appear in the terminal log when I rate an image or assign a color marker. Also no errors when XnView exits.
I'm open for investigation.

Cheers,
Konzertheld
(Edit: Initially, I wrote "categories" but I meant color markers.)