It's possible that the thumbnail has become disconnected from the original picture. Let's see:
A. What happens when you try to Edit one of these pictures in Photos? Do you get an error, or can you crop and enhance the picture?
B. In Photos>Preferences>iCloud, do you have iCloud Photos checked? If so, do you have "Optimize Mac Storage" checked?
C. In Photos>Preferences>General, do you have Copy Items to the Photos Library checked?
Here are some things you can try--they may help, or the result may help figure out what's wrong:
- Restart the computer (of course)
- Repair your Photos Library by closing Photos and re-launching by option-command-clicking the app icon
- Make a new user and see if the same problem recurs with it
- Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
- Start in Safe Mode (different for different computers)
Start up your Mac in safe mode - Apple Support