Behaviour is completely inconsistent - and on both iPad and iPhone. Like you I've tried all the recommendations; sometimes the turn auto off and set to minimum gets it working but not always - so I mostly leave auto turned off, but even more bizarrely when auto s off, it sometimes turns the brightness down to minimum so I can't see the display!
It's made the iPhone more or less useless as a Sat Nav, because the brightness goes down (by itself) to too dim to see while you're driving along and can't fiddle with it.
It was working fine before - so much so that i didn't even realise there was auto brightness functionality - it used to do what Apple stuff is supposed to do - it just worked without me thinking about it. But alas not any more - broken - Apple please fix!