When I play games or shoot videos more than 15min, the maximum brightness drop up to 50% that I can not increase the brightness. I had to wait to restore the brightness since I cant see the display clearly while I play games. I visited Apple store for 3 times so far. They tested my iPhone X but there are no hardware issues. Of course, I turned off all auto brightness stuff. So they decided to replace OLED but it still happening. I thought third party apps causing this issue but Apple apps like Camera also causing this. If you dont know what Im saying, try to play PUBG or into the dead 2 for 20~30min then you will see what Im saying. Does anyone have similar issue like mine?