Pixel owners are now reporting of a bug that isn't allowing them to unlock their phones using PIN codes. Mostly, Pixel XL owners seem to be affected by this bug, but Pixel, Pixel 2, and even Pixel 3 owners are also reporting of the same issue. The issue seems to be affecting phones running on Android 9 Pie and Android 10 both, and it effectively makes the phone inaccessible. One of the workaround is that you switch to Pattern or Password authentication for the time being.
Several Pixel owners, mostly of the Pixel XL, are reporting on the Google forums about this issue. Every time a user tries to unlock their phone using a correct PIN code, the display turns black forcing the user to activate it using the power button. This brings them back to the lock screen, wherein they are asked to put the PIN code again. This goes on in loop, keeping users locked out of the phone. Users need to then authenticate fingerprint to unlock their phone, and this also doesn't work when the phone is rebooted or unlocked after a very long period of time.
Many users had to do a factory reset on the phone to get it back into working condition, but this isn't an ideal solution. Google hasn't responded to the issue as of yet, and there's no clarity if they are aware about this lock screen bug.
If a factory reset seems to be an extreme measure, one user on the forums page, has posted a workaround. He says that when you get to the PIN code lock screen page, click on Emergency call > press the volume buttons > Press the Settings icon on the side volume bar, and then put in your correct PIN code. A new pop-up saying that ‘System UI keeps stopping' should show up. Click on ‘App Info' and then you should be able to unlock using your fingerprint. However, it does only let you access to Settings and create a new user ID as well. This will allow you to use the phone and update to the fix, whenever Google decides to roll it out.
The issue seems to be affecting Android Pie users on the August patch, and Android 10 users on the September patch. The user who came up with the workaround also says that the issue is "with the unlocking security (authorize Part) of the Pin itself as you can't seem to remove it in the settings as this crashes the Settings app."
In any case, switch to Pattern or Password unlocking for the time being, and if your unit is also affected, let Google know through the support page.