This information is recorded in the notes to release the Google Play service update version 25.14, security & privacy section.
Basically, the restart of the device will bring the camera back to Before First Unlock (BFU) - ie before the first unlock. In this state, the data on the device is encrypted and it is difficult to be illegally accessed if the user has not entered the opener.
Meanwhile, if the device is in a First Unlock (AFU) state - that is, unlocked and used, the data is more likely to be exploited when bad guys have physical access to the device.
Some phones like Google Pixel also display notifications right on the lock screen that users need to "unlock to fully use the features and data" after the device has just been restarted.
Google Play service version 25.14 will be released in the next 1 week. However, the above feature may need a few more weeks to activate officially on Android devices.
Google has not yet published further details about any specific Android versions that are supported, or users can customize this feature. It will be more reasonable if the device shows the notification to the user whenever the process automatically restarts.
