Try going into your phone's "Settings" and check to see if "Auto Brightness" is enabled. If it is, this could be what is causing your issue. I would try turning it off and see if that resolves it.
Try going into your phone's "Settings" and check to see if "Auto Brightness" is enabled. If it is, this could be what is causing your issue. I would try turning it off and see if that resolves it.