Photos "People" Function Keeps Merging People
Talking to Apple Support, they said the only thing they could come up with was to delete my Faces database. That hurts -- it involved deleting 116 people covering 170,000 photos. All of my devices are still "processing" after several days, but I've been adding many faces back in the time being. 

One helpful comment from Apple Support was to make sure to ignore the automatically generated circles around faces if they covered multiple faces and simply to draw new circles (on Photos for macOS) that only cover the face in question. Previously, if it drew an automatic circle, I would label that with whichever person was most prominent and draw a manual circle for the secondary person in the circle. (This isn't as helpful on iOS where it lacks those circles and the ability to add manual faces.)