thumbnail not same as photo

Try to rebuild the thumbnails by rotating them back and forth. That will create new thumbnails.

You can select multiple thumbnails at once, enter the key combination ⌘R, and then ⌥⌘R to rotate them back.

Rotating is a lossless edit and can be done while viewing the thumbnails. But don't select all thumbnails at once, try it in smaller batches.

See this user tip: How to Rebuild Thumbnails in a Photos Library

https://discussions.apple.com/docs/DOC-8521

I have found no way to select multiple thumbnails

You select multiple photos like this:

• Select a range: While viewing the thumbnails in the moments or in an album, select the first thumbnail with the mouse, then hold down the Shift key and select a different thumbnail. All photos in between will be selected.
• Select multiple photos individually: Select the first photo, then hold down the command key ⌘ and click some other photos. All clicked photos will be selected.

You're welcome. 🙂

Multiple selection is working like in the Finder, but not documented in the Photos Help. At least, I could not find it.