iCloud Photos Syncing to a Device Even After Logging Out

In September 2023, I logged out of my iPad (both AppleID and AppleID for Media and Purchases) to give the iPad to a family member. They logged in with their AppleID for both, however, my entire iCloud photo library kept syncing to the iPad, mixing with their photos and becoming a part of their iCloud photo library, syncing to all of their devices. I tried restarting, logging in and back, logging in and force removing that device from my account to no avail.

I wiped the iPad, and only then, setting it as new using their account, fixed the issue. Until yesterday. They updated the iPad to 17.5, and right after the update my pictures appeared on their iPad. There's absolutely no connection between my account and that iPad, it was wiped clean, yet the pictures still appeared.

This is both creepy, awkward, and a privacy violation. I can imagine how many people sold their Apple devices without fully wiping them, just by logging out and deleting everything (which is apparently not safe). And since even that did not help, how many people now might get other people's photos showing up randomly on their devices?

For context, I am a software developer and I spent years working in tech support. I understand how it works, and I checked out countless guides from Apple to make sure I did not miss anything. Everything was wiped, every connection severed; yet this still happened.