Adaptive brightness isn't functioning properly.

# Rebellion: Install "sensor list" app to ensure that the ambient light sensor is working properly.:https://play.google.com/store/apps/details?id=com.miian.android.sensors&hl=en

1. Please try to use the device in a different light condition or environment.
2. Open Settings -->Display to check whether Reading mode or Night mode is enabled.
3. If the issue still persists try to clear cache, back up all the data and perform a factory reset.