I guess your file manager may be broken.
Maybe you try it with this posting of mine, where I describe different methods to get Krita files with thumbnails:
via Krita plugins,
via fixing Dolphin and a link to the description on how to do it, if you should use it as your file manager,
via links to other possible solutions,
I guess you’ll find something that will help you further, and if not, then please report back and name the exact version of Krita and the kind of installation e.g. repo, Flat Hub, Steam, AppImage you use. Also describe briefly what you tried so far.
Michelist