For all android smartphones that doesn't have ambient light sensor and using front facing camera as virtual light sensor, it will create problems with auto brightness specially if there are smudges or even dust particles in front facing camera.
Moreover if you turn on auto brightness for these phones, it consumes significant amount of battery and tops in the battery consumption in chart.
In previous device that has auto brightness there used to be a sensor that is dedicated for ambient light sensing. Those device hardware for auto brightness was much more efficient compared to using front camera for doing this job.
When front camera does this virtual light sensing, camera needs to be switched on frequently, and these camera module power consumption is much more than even screen can consume which supposed to be the most power hungry component of any phone.