Face-name assignment in macOS Photos app is only local, not synchronized to iCloud

Apple's photo management software is a great illustration of Apple's decline.

Anyway, just so you know, the assignment of Face/Pet identification to a Photo is only stored locally. It's not synchronized to iCloud.  I have also read that it does not really work with storage optimized macOS Photos Libraries; the process only works on full res images (might very very slowly work, never really catches up with adding photos).

If your Library is corrupted and you start a new instance from iCloud you lose all the Face ID information (and probably other information). I don't know if full local backup and restore saves the Face identification data.

I could test this because I use macOS Photos with both an optimized and full res Library.

For images that I care about I use a smart album in my full res Library to tag Face identified images with keywords. Those are synchronized.

PS. Bonus bug just discovered today: Photos.app has keywords that don't find any photos using a smart album, but when you delete the keyword you are told there are actually photos that have it.