I was organizing photos on iOS 12.2 and it reported that one of the people in my library had "new photos" it wanted to confirm. One or more of those had that person cheek-to-cheek with another person in my album (let's call them Joe and Jane). I was horrified -- since I have thousands of photos of both Joe and Jane -- to find that as soon as I approved that ill-fated photo, the facial recognition system decided the two people were the same person and merged their entire set of photos.
I tried deleting both Joe and Jane on macOS 10.14.4 and recreating them by manually tagging them in photos where they were by themselves and not together, but no matter what I do, as soon as
photoanalysisd kicks into gear when Photos is closed, it merges the two people back together into one entry. I can manually label photos with Joe's name and I'll still come back to find them relabeled as "Jane." I can select all of Joe's photos in Jane's people entry, right click the photos and click "Jane is not in this photo," have Jane's album only contain photos of Jane and within minutes those very same photos will again be labeled as Jane (not just other photos, the ones I've manually corrected Photos on!).
I'm guessing Photos' machine learning database is corrupted, but I'm not really sure how to completely delete data on Joe and Jane (beyond what I did in deleting their entries by selecting all the photos in them and using the "Name is not in this photo" option), but somehow I need to convince Photos to either start from scratch or believe me when I tell it that it is wrong on its labels so it doesn't keep overriding me.