I've been using Painter for a few years, and haven't had trouble.
This year I upgraded from 2012 to 2019, and then again to 2020 when it came out.
When I upgraded from 12 to 19 nothing went wrong.
After upgrading from 2019 to 2020, all my .rif files in my explorer window, and when opening a file in the program are the same painter thumbnail icon!
I have a very chaotic filing system, and I relied heavily on the previews for sorting. (I'm going to fix it, promise!)
I've misplaced a file I need, and this is really frustrating that a program built for visual people won't show a preview of the image.
I'm running Win 10 home. I have both 2019 and 2020 installed
(because I'm slowly learning a new workflow in the new program.)
I have exactly the same problem, and I really NEED those thumbnails to show up ):