This might be the stupidest thing I've ever said, but is this how auto-brightness works?
In high light conditions (sunny day), the iPhone screen gets dimmer.
In low light conditions (dark room), the iPhone screen gets brighter.
So, unless I'm completely crazy, the software is not designed to dim the screen in a dark room. Ever. Even if it's set to full brightness.
When set at 50% the auto-brightness works best because it has the room to change the brightness based on lighting conditions.
Try sliding auto-brightness off and back on?
Sorry if this was stupid but that's all I got. I've just never seen my iPhone dim in a dark room or get brighter in a light room.