Howdy, Eric~F
To answer your first question, no, no one else has access to the laptop.
I am running Monterey 12.1 on a 2020 MacBook Air with the M1 chip
After reading your reply, I restarted my Mac all but one stayed the same. One of the photo icons was simply gone and I had to add it back. I haven't been on it since to see if any have changed or also erased.
I have five users set up currently (all me). I sign in to whichever based on need: travel, goofing around, side hustle. etc. and it keeps each account uncluttered. I can easily and quickly jump between accounts. The laptop just randomly swaps the user image I've selected (or deletes it). It doesn't affect the accounts at all but it's annoying on a personal level and I want to figure it out, haha.
Thanks for taking the time to respond and I'm happy to try something if you've got suggestions.