Because Apple's mobile phone will have an automatic screen adjustment function, that is, the screen brightness will be automatically adjusted according to the ambient light. If the light sensor is blocked, the system will mistakenly think that it is in a gloomy scene, so the screen will be darkened.
One way to solve this problem is to turn off the automatic screen brightness level and enter Settings-Display and Brightness-Auto. Another method is to clean the dust on the Face ID module, because this module has a light sensor, so it is best to keep it clean.
If the above methods can't be solved, you can go to the Apple Direct Store to solve it with the clerk.