Did anyone notice, that the Big Sur Photos-App renamed all images in the "masters" folder from their original name at the import of the file to just a combination of numbers? For example "holiday.jpg" to "547892-773624-992772.jpg".
I had to downgrade from Big Sur back to Catalina and I (of course) couldn't use the photos library any more. Fortunately, the files still have the original name in the iCloud. That means, I could delete the local library and download everything from the iCloud.
Everyone with specific image names and no iCloud backup will have a hard time to move the photos library from Big Sur to Catalina or lower.
I had to downgrade from Big Sur back to Catalina and I (of course) couldn't use the photos library any more. Fortunately, the files still have the original name in the iCloud. That means, I could delete the local library and download everything from the iCloud.
Everyone with specific image names and no iCloud backup will have a hard time to move the photos library from Big Sur to Catalina or lower.