If the Automatic brightness doesn't work on the Redmi 12, please refer to the following steps:
1. Ensure that your phone's operating system is up-to-date. Software updates often include bug fixes, security patches, and improvements that can address software-level issues;
2. Check if Auto brightness is enabled by going to [Settings] >> [Display] >> [Brightness level] >> [Auto brightness];
3. Restart your phone to refresh the system;
4. The phone case and toughened film can interfere with the sensor module, please remove them if you've used them;
5. The feature relies on the Ambient light sensor. Check if the sensor is in good condition by going to the [Phone] app and inputting *#*#6484#*#* or *#*#64663#*#* to enter the hardware detection (CIT mode). If the hardware test fails, please contact our local official service center or your seller for further assistance. They will help you in this regard;
6. If the above steps can’t resolve the issue, please back up the data, log out your Xiaomi and Google accounts, and perform a factory reset.
However, if there is no hardware issue, we hope you could submit user feedback by going to [Settings] >> [Services & feedback], and wait for the future updates.