This happened to me this evening, we came back from holiday and I'd left my iPad plugged in at home (we use it as the HomeKit bridge), I noticed the bridge was unresponsive, so I went to unlock the iPad and saw a notification that the OS had been updated to 12.something, it wasn't connected to Wifi any longer and when I went to tap in the passcode it's not recognised?
After some tinkering, I went to unlock my iPhone X and that too requested the passcode, which I haven't changed in months and it's not recognised either (incidentally the iPhone is MDM managed by my work) ?!
I'm a bit pissed off as I need my iPhone for work (RSA SecureID soft tokens), and I haven't had a chance to sync my holiday photos from it yet