As a test launch Photos with the Option key held down and create a new, test library. Import some of the same photos and test to see if the same problem persists. Does it? (This tells us if the problem is limited to your current library or is more wide spread)
If the problem persists in the test library log into another user account, Set up users, guests, and groups on Mac, and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).
Next, if you do not have the iCloud Library enabled launch Photos with the Command+Option keys held down and repair the library. Test to see if the problem persists.
Be sure to have a current and full backup of your library before running these tests.