Activation lock kicking in after months

I'm a developer and I bought a 2nd hand iPad for development aid in June 2019.

It came with the usual Hello setup screen, so I put my email and password and evertything was fine.

I was able to use iCloud, was able to debug my app on it, everything was normal.

In app purchases was difficult to test because my developer accounts grants me free subscriptions on my own apps, so I created another iCloud account and used on this iPad (and also on an iPhone 5 I have for the same purpose). Again, everything worked fine.

Last week, the iPad updated itself to iOS 12.4.6 and, after that I have used the reset function to clear it (I wanted a clean first use experience for some new developments I'm testing).

Now it keeps telling me this device is locked and asking for an e-mail/password that is not mine (as it masks the e-mail, I can't even tell who owns it).

So, what happened? This iPad was stolen before I bought and I just have luck setupping in the first place?

The iPhone Finder doesn't show it on my account (although it appears on my iCloud devices list, but the only action I can do here is to forget it)

I'm having trouble understanding if this is a mistake or anything else (if I never could pass the activation, ok, I could do something when I bought it, but the activation kicking in just now is insane!)