If you already disabled auto brightness, then that means your phone is getting hot and reducing the screen.
You can try the hidden menu settings and turn on a couple things but that could make your phone overheat.
Do you have overheat problems now?
No overheat problem...I set brightness around 75%...
If it's not overheating, try turning the auto setting back on, rebooting, then turning it back off. And maybe reboot after that step, too.