When I turn the screen on my display is auto dimm the brightness even I unchecked auto brightness...Thanks.
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.