Hello!
Yes, it happens in Photos app at my iPhone SE 2nd Generation. If I sign into my iCloud via pc, also I see all her pictures.
She did never entered her Apple ID at my device and this started to happen few months ago, at least is when I realized.
We use Family Sharing. I’ve contacted support and the solution was to manually search for my wife’s pictures (among 20K pictures), download them in the PC, delete them from my iCloud and upload them back to my wife’s iCloud, as she does not have them anymore at her iCloud.
Also happens with her albums. I have her Instagram photos album and I do not even have an Instagram profile, or the app.
Thank you!