Iphone Xr Screen Dimming

My brand new iPhone XR keeps dimming. Auto Brightness is turned off and anything that could affect my screen brightness is off. I will be using my phone when randomly the screen dims drastically and the brightness says its max. After a while my phone will go back up when ever it feels like it, I have read a lot of people have this issue, should I go to the store and get it fixed, I have apple care. Very upset, paid 900 dollars for a phone that doesn't even work properly. Sometimes my phone lags too, like had the phone not been so expensive I wouldn't mind, but I feel robbed.