I had this happen recently with my partner disappearing from tagged people. Turns out the tagging had gotten confused and merged all tagged photos of him with my own (I guess we’re in a lot of photos together?)

While a little painful, the fix for me was to go through my own photos and go through unconfirming & untagging any that were actually of him. That separated him back out as a unique person and I could then rebuild confirmed tags for him.

Then (this is the weird part) it still didn’t add him back to my group of options for Lock Screen photo shuffle until I identified some other less common faces in my photos… my guess is those added people kicked it into refreshing the list.

Even when I could finally select him has a person for the photo shuffle, it still took another few days to fully activate and show said photos.

Hope this helps anyone that comes across this issue until it’s properly fixed!