Try the following in order:
1 - launch Photos with the Option key held down and create a new, empty library. Import some photos into the library and see if the problem persists. then ope your original library and test. If you problem persists go to #2.
2 - log into a new admin account and ope the problem library. (Do not open any other apps) See if the problem persists. You can delete the account when you're finished. If the problem isn't present in the new account that means you have some 3rd party software that may be affecting the Photos library. In that case try #3.
3 - boot into Safe Mode from your original account according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.
NOTE 1: 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
NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.
Post back with the results.
