Pixel users are suffering from a lockscreen loop problem.
Some users have hit the Pixel forum to share their experiences with the glitch. The issue occurs when a user tries to unlock their handset with a pin. When the user inputs the correct pin, the screen turns black and then goes back to the lockscreen.
Users can bypass this if they use their fingerprint to authenticate, though that won’t work after restarting the device.
One user has tried to force a failure in the System UI by taping on the emergency call dialer and then navigating to the volume menu. Eventually, this would allow the user to get into the ‘Settings’ page. From here, the user was trying to add a new user. However, it might have made more sense for the user to change the lockscreen type from pin to a pattern.
In all cases, the Pixel owners use PIN codes for authentication and are on Android 9 Pie or Android 10. This mostly affects the Pixel XL. However, other Pixel 2 and 3 devices have also experienced the issue. Product experts say to reset the device, which likely isn’t an option most users will want to take.
If you’re experiencing the problem, you can get help from the Google support page.
Even if you’re a Pixel user and you’re not suffering from the issue, it’s probably best to change to a pattern unlock before you start encountering the problem.
Source: Pixel Phone Help (2)