The krita thumbnailer normally used, found online or documented doesn't make previews for file names with a space
Ex: test file.kra(won't have a preview) testfile.kra(will have a preview)
Solution Update the thumbnailer and clear cahed failed thumbnails:
Why this solution, I believe the %i and %o variables aren't being passed properly when they have spaces
I tried it without them, cleared the thumbnails .cache and it doesn't work without the quotes around those