I opened the photos.app in Catalina today and noticed that most thumbnails appear as green pictures.
Upon double-clicking it show the image correctly, so I'm thinking maybe something is wrong with the caching, but deleting both the
com.apple.Photos.plist as well as
~/Library/Preferences didn't do anything.
Update: I noticed that a lot of my portrait photos are a bit messed up as well (they are all blurred out), but clicking on edit fixes the photo, but not the thumbnail