I have the same problem for about two years. Tried everything, reinstall the Mac, redownload all library from icloud, etc. etc. anything and everything you can think of.
Finally contacted Apple several times and had different people from the Apple development team (not the regular support team), do several scheduled sessions with me where they connected to my Mac, toggled some special profiles, and collected some logs and debugging info from Photos.
After around one month, I got a call from one of the senior technical team saying that they had identified an issue in the Photos library that prevented to scan of people's faces and the team would eventually work on it. They didn't say when it would be released and they said that Apple doesn't give estimations or deadlines on such fixes. I was trying to get details but the most I got was "If it's not fixed in some months by an update, call us again". But around a year has passed and they never fixed it and I don't want to do all the lengthy debugging process again...
I just resigned and forgot about people scanning