Phone dims with auto-brightness off & True Tone off

My iPhone SE dims in an auto-brightness fashion when I don’t want it to. It dims even with auto-brightness OFF, True Tone OFF, power saver OFF, and phone NOT overheating. It has done this since first purchased. The phone has been turned on and off, and it has been updated. I want the brightness to stay at max brightness, but it simply won’t let me and always adjusts the brightness.