I have found related topic. Check this out.
https://developer.apple.com/forums/thread/652691
anton_makarenkov
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macOS 10.15.7 Xcode 12.1 have same problem
As for me on simulator iOS 12 fetchThumbnails called each time i delete app and remove file provider from Files apps. On device this doesn't work. But when i restart device as u write above, fetchThumbnails start calling. Thank you a lot