We're not using the same local user account.
I suspect this is what I've done though how I've selected to leave the local data I've no idea. Obviously I know how I did it but not why

I should have known better.
It's the only thing that can have happened, I think.
Having said that how on earth do I unravel it? If I delete her passwords from my devices won't that delete them from her devices? And vice versa. Maybe not because they are from her appleid?
And all this on 3 devices each

I haven't tested that yet.
Thanks for your experimentation