I have an iphone 7+ and sometimes the brightness setting will be all the way up but the brightness will only be 1/4 of the full brightness?

I have an iphone 7+ and sometimes the brightness setting will be all the way up but the brightness will only be 1/4 of the full brightness?

That happens when the phone is heating up, usually through 'hard running' for lack of a better term. As it cools down it would gradually restore the full brightness.