I have the same situation with iphone 8 on the latest iOS. Auto brightness turned off, True tone could be off or on (doesn’t matter). So, brightness could by itself suddenly switch to 75% or lower, to 50%, however brightness setting set to max. It happens while gaming or watching youtube etc. Brightness switch from 75% to 100%, for example, doesn’t increase brightness, it stuck on 75%.
I have had such issue with my 6s and the problem was in certain iOS version, Apple released it last year for 6s to fix suddenly turn off issue (my iphone 6s was eligible for free battery replacement due to this) and there was a dependency with battery %. At 75% battery screen dims to 75% brightness, at 50% to 50% and so on.
Just now, writing this comment my iphone 8 was at approximately 75% max brightness and I couldn’t set it to 100%, it also was connected to the power from 60% battery, at the end of the comment iphone increased brightness to 100% 2 times and came back to normal, battery charged to 73%.
Considering this situation I can assume that this is iOS issue and made to prevent from overheating. I don’t like such approach, instead of doing everything well, Apple trying to fix the problems in a strange ways. Disappointed.