That’s not how auto brightness works.

Auto brightness uses light sensors to adjust the brightness up or down from where you set it based on how much light is in the environment you’re in. It does not adjust based on what app you’re using.

For example if you have it set at 50% in a dimly lit room and go outside in bright sunlight, it will turn the brightness up to compensate.

The faster way to do what you want is to just use the brightness slider in control centre (pull down from top right) before/after playing your game.