Screens brightness decreases during game play

I use this amazing phone for mostly for gaming only, no phone calls, no texting, no emails, no social media. I have two games downloaded, RR3 and PubG mobile. 99% of the time I am playing PubG mobile. Sometimes during game play I have been experienced the screen display will sometime decrease by a noticeable percentage. It also does this during RR3. I mostly use this phone for gaming and I do play with the phone charging due to battery drainage. I have changed in developer settings to STAY AWAKE that the phone should not sleep while charging. The brightness is at 100% and adaptive brightness is off. I am using the game booster feature and all notifications are set to off (do not disturb.) I have watched movies on Netflix and have not experienced this issue, Netflix is not added to Game Booster. Game booster configurations is the following. CPU: 2.80 GHz Resolution: 1440P FPS: 120