Google’s Pixels may be the smoothest Android phones, but they haven’t been the most reliable with one QA issue after another. With the Pixel 3a and 3a XL, Google doesn’t buck that trend.
The Google Pixel 3a and 3a XL have reportedly been subject to random shutdowns for no apparent reason. These shutdowns only occur when the device is not being used, and users need to restart it by holding down the power button to force a soft reset.
While not interrupting active use mitigates some of the more annoying concerns (such as shutting down in the middle of an important call or message session), shutting down when the device is inactive means a user could miss that important call or notification and not even be aware that there was a problem without picking up the phone.
A per Android Police, it is not currently known exactly what’s causing the issues and no fix is currently being issued as a result. If this is truly a widespread issue, Google will most likely fix this with a software update within weeks.
Source: Android Police