Naming People in Faces

I have a 2019 Macbook Pro running Big Sur. I migrated all of my photos from my prior Mac Book. Aside from the fact that Photos (in my opinion) is far inferior to its predecessor, iPhoto, at detecting and suggesting names for faces, when I do tag a face it sometimes will only assign the contact's first name even though the person's full first and last name appear in the drop down menu under the face detection bubble. If I go into the faces album and select that person and open up their individual album I can then click on their blue-text name in the upper left hand corner and change it to first and last, but it will then tell me that the name has been changed, but when I close the dialog box it still shows first name only. The even weirder part is that for some "first name only" people it will accept the change but I can't seem to find any rhyme or reason to it. So when I open my Faces album about half my people have first and last names and half have first name only. I know for a fact that when I identify them in individual pictures I use first and last names. It is super frustrating. Oh, and it's been updating my faces "when you're not using the app and your Mac is connected to power" for an eternity, but that's another headache.