Hello,
Few months ago Apple released a more modern version of their cloud sync service for windows that doesn't just store everything on disk, but also supports streaming files.
Weirdly, files under iCloud Drive directory only show the thumbnail whenever they're not synced (so either are not downloaded yet, or not uploaded yet).
After they're synced - this file has no thumbnail in Everything, but the thumbnail in Explorer is still there.
It's kind of counterintuitive if you ask me - I'd think the images that aren't on the disk yet would have problems, not the downloaded ones.
I did check on latest stable and latest beta version of Everything, as well as on a fresh Windows 11 install.