My Photos just forget people eventually

One of the tricks I learned for user léonit is to add a keyword for each person in a photo. That way you can do a keyword search for them instead of relying on the Faces/People feature.

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and then reboot normally and check to see if the problem persists.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files