Suggestion:
If you use your phone for a long time with high intensity, you may encounter the situation that the screen brightness automatically decreases after the temperature of the body rises to a certain level, and you cannot brighten the screen even if you turn off the auto-brightness.
This is one of the security strategies of MIUI system, which can effectively control the phone temperature to avoid overheating and prolong the life of the screen and battery. This is also one of the common solutions in the industry, not a phone failure, please feel free to use.