Apple ID on new iPhone changed to Apple Id of old iPhone's new owner. How did this happen?

This is bizarre. I purchased a used iPhone 12 mini phone that had been reset and wiped clean and installed the new glass edition of iOs mid September 2025. I then setup this phone by migrating from my old iPhone SE 2020. I needed my Apple Id to transfer to the new phone and wipe clean the old phone of course. All good. I then sold the wiped clean old iPhone SE to a young girl who is a family friend and she set the phone up with her Apple Id. A short time afterwards I looked at the account info on my new IPhone 12 mini and it was now using an unrecognized Apple ID. Turns out it was the Apple ID of the young girl who I sold the iPhone Se to and from which I had transferred data to the new iPhone 12 Mini! How can this possibly happen? The girl is very young and never once had my new phone in her possession and certainly didn't know my Apple ID let alone the password. When I went to sign her out of the new phone I was asked for her Apple Id which I of course never had. It was only after retrieving the old phone from her and getting her Apple Id and password, and with the two phones in hand managed to transfer the new phone to my Apple Id. The password for her Apple account didn't work either, but she is young and wasn't 100% sure of what it was but we managed to reset the password and through the "forgot password" process managed to fix the problem. What would have happened if I had sold the old phone to a complete stranger? I would have a new phone that I couldn't do anything with. Has anyone had this same issue? This is definitely some kind of Apple software glitch. Thanks

[Re-Titled by Moderator]

Original Title: Apple ID on new iPhone changed to Apple Id of old iPhone's new owner. How did this happen?