de0u
I've witnessed another 2 instances of 2FA unlock without PIN prompt, and I'm confident in these cases it was not the result of the screen timeout behavior noted above.
First instance today: Phone was locked and on change with a wall socket over night when I picked it up to check a stale notification this morning - 2FA worked as expected. However, when I went to unlock the phone again 2 hours later (still connected to wall plug) to dismiss an upcoming alarm, the PIN prompt failed to appear when presented with a fingerprint.
Second instance occured 5 mins later: While still plugged in, I received a notification and picked up and unlocked my phone to check it. Again, unlock occured with fingerprint only.
When I tried to replicate this a 3rd time, 2FA worked as expected
I'll report back if I see any more apparent instances 2FA-unlock without PIN prompt, with the hope of pinning down what might be triggering them.