missing photos in photos library

Check in your photos if you have a recently deleted file in the side bar. Also in view at the top click on hidden photo album. If it says hide hidden photos album then nothing there.If you're missing photos or videos in the Photos app - Apple Support You can also rebuild the photos app by holding down the command option keys while clicking on photos.

Is it possible iCloud has been turned on on Jan 27th (eg by an update).

Failing that I would try the following:

1 - Try booting in safe mode - does it still happen? Then reboot normally and try again.

Start up your Mac in safe mode – Apple Support (UK)

Safe mode will clear temporary files and caches, and sometimes fixes odd glitches such as this.

2 - Create a new empty library and try with that library open (hold down the option key while starting photos, and click "create new". To switch back to your current library afterwards, do the option key start again) You'll need to import a few images to that library to test with.

3 - Try creating a new user account and try when logged into that account.

These checks will tell us if the problem is system wide, or linked to a particular library or user account.

Photos may be losing the connection to your current Photos Library and continually switching back to an older version of your Photos Library. Do you have any external drives connected to your iMac? Then check, if photos has imported the photos accidentally into a different library. Launch Photos while holding down the options key ⌥ and open all libraries in turn, that will appear in the library chooser dialog.

Some time ago I noticed, that Photos would automatically open a library on a backup drive instead the one I wanted to use.

Hi

Are you syncing with any other devices using iCloud, and deleting them on the other device?

Alternatively - where is your library stored - if on an external drive, what is the format?