People names in Photos

Can someone please explain how people names in Photos works? Some of my photos are tagged with first and last names from my contact list. Others have only first names, and when I try to rename them to match the name in the contact list, they persist with first names only.

Other times, I’ll try to add a name to a photo, and the proper name will appear after I type a few characters, but after I click on the name, I’m left with a photo named the first few characters.

My grid of photos in the people album is alphabetically ordered, except where it isn’t.

ALL of my devices keep telling me that Photos is scanning faces in the background, but the process NEVER finishes. It’s been YEARS.

WHAT IS GOING ON?

Can someone please help me or recommend a macOS and iOS app that actually works when it comes to faces and names?