Photos sharing with family members

To preface - my family and I have had a 4 person family iCloud sharing account ever since it was a thing. We have never had any issues with it.

when my father got a new phone, i got his old one. He completely wiped it and i uploaded all my stuff accordingly. A few days later i had gotten 30k photos of his onto my phone - i have no idea why. I have switched to many old iphones of his and never had any issues. Suddenly months later, with no one change the settings of anything, MY photos started appearing on his new phone gradually. He still is saying he is getting more. This is not occurring with the other 2 family members. I don’t think family sharing is the issue as i removed myself and it was still happening. We all have our things backup up to the cloud - never have had this happen. How can we fix this ? We do not have a shared library set up. There is nothing in settings that is related to this. We have never put each other iCloud IDs on each others phones