Fortnite screen brightness issues/dimming after 20mins

Playing Fortnite at 3/4 brightness and after 22mins it automatically reduced the screen brightness to about half way maybe slightly less!! I was not even playing it at full brightness!!!

3 mins after of closing the game full brightness was possible.

I had the iPad plugged in with 100% battery, however I tried it on 100% with no charging still the same.

My auto brightness is off, white point is off, zoom is off, contrast is on.

I have erased all settings and data and set it up as a new iPad with new Apple ID.