Thumbnails for old photos are wrong in Photos app
To Rebuild the Photos.app Thumbnails:

Backup your library before following these instructions.

Delete the old thumbnails and various parts from the library:

Quit the Photos app.

Open Finder, and go to the Photos.app Library, on my system it is called Photos Library and is found in the Pictures folder.

Now right click the library and click Show Package Contents.

Next delete the folders called Previews and Thumbnails.

Then delete the files called ImageProxies.apdb, ImageProxies.apdb-wal, Library.apdb and Library.apdb-wal from the database folder.

Lastly delete the folders RKCloudResource, RKImageProxyState, RKMaster and RKVersion from the recovery folder.

Now rebuild the library

Open the Photos app while pressing ⌥+⌘. It will ask you to if you want to repair the library, click Repair. It will do a repair, then your photos will go through a reprocess/reimport process. Once finished the thumbnails will start rebuilding.

P.S. I tried this on my own Photos library, and it worked.