iphone 12 pro max screen dims very much

My iphone 12 pro max screen display dims on its own. most commonly when i use it to fly my dji air 2s drone, which is always charging phone. I contacted apple and had them send me a new phone after a trip to the apple store. the 2nd phone does same thing, i was in the shade today flying and the display dims once, then more with additional use. This makes the phone a worthless tool for its intended purpose.

I only wish apple would have admitted the fault so i didn't have to go through the hassle of getting a 2nd phone.