Why does my screen always go dark (iPhone)?

Especially when I play Roblox (don't ask hahah💀), my screen gets darker and darker, and it's really annoying. I've already turned off auto-brightness in the settings. When I look in the control center, the brightness is still all the way up. If I close Roblox, it stays dark for a while and then eventually gets bright again.

Does anyone know if I need to adjust anything else so that the brightness doesn't change at all?