In Settings > Display & Brightness, the Auto-Brightness option has been removed in iOS 11 and set to on. This is a terrible decision. Auto-Brightness has been poor on any device I have ever seen it, and iOS has always been particularly bad. It is inconsistent and over aggressive. I appear to have a choice of it being too bright for a bright room, or too dark for a dark room. Except for the occasions where it gets it completely wrong. I was in a dark room last night, but the screen was massively bright. Maybe it sensed the light source, not the ambient light, but I don't know. All I know is it is a terrible decision.