The automatic dimming of the screen of Xiaomi phones is usually caused by the automatic brightness adjustment function. Turning this off prevents the screen from automatically adjusting its brightness based on changes in the light of the surrounding environment. Here are the steps to turn off auto-brightness:
Open Settings
On your Xiaomi phone, find and open the Settings app.
Go to the display settings
In the settings menu, look for the "Display" or "Display & Brightness" option and tap to enter.
Turn off auto-brightness
In the display settings, find the "Auto Brightness" or "Adaptive Brightness" option.
Turn off this option. Usually, this can be done by sliding the switch next to it.
Adjust the brightness manually
Once the auto-brightness is turned off, you can manually adjust the screen brightness. On the same page, there should be a slider bar that allows you to adjust the brightness to your preferred level.
Check the power saving mode
If the problem persists, check if Power Saver is enabled. Sometimes, the battery saver mode may reduce the screen brightness to save power. You can find and adjust the battery saver settings in Battery & Performance settings.
Update the system
Make sure your phone is running the latest version of MIUI. Sometimes, a software update can fix these kinds of issues.
If the problem persists after completing the above steps, it may be due to a software failure or hardware issue. In this case, it may be useful to try restarting your phone or contact Xiaomi customer service for further assistance.