Some of My Albums now appearing in my wife's Photos app

I have several albums in Photos under "My Albums" (and several under "Shared Albums"). A few of the "My Albums" albums have now appeared in my wife's Photos app under her "My Albums". Some only have a few of the photos in them that I have in my version. I've not done anything with albums recently.

We are signed in under our own respective Apple IDs

We are part of a Family for sharing apps etc.

We haven't activated the iCloud Shared Photo Library feature.

Unless I'm missing something, something has gone badly wrong in Apple's infrastructure behind the scenes. Very annoying.