User ID photo/icon changes randomly

I got my new macbook pro w/M1 pro about a month ago, and twice already my logon icon changed by itself. The first time, which was a day or so after setting up the new machine, it changed from the photo I've been using for many years to one of the generic icons (soccer ball if I remember correctly). So I changed it to one of the built-in animated character icons that I preferred. It's been fine for the last 3 weeks or so, and today, I find it changed to my son's pic. I asked him about it and he said he changed his user icon on HIS iphone, but didn't touch any of my devices. How did his changing his photo affect my icon? Is this going to happen any time someone in my family changes his/her iphone user icon? More importantly, how to do i stop this from happening altogether? Thx