Hello! Based on the information you provided, it seems that the issue with your back camera started after updating to MIUI 14. This could be due to a software bug or compatibility issue with the update. However, since you have already tried troubleshooting steps such as force stopping/clearing data, installing a 3rd party camera app, hard resetting, and downgrading the software, it is possible that the issue is hardware-related.
If the back camera still does not work after performing all the troubleshooting steps, it is recommended to bring your phone to a professional technician or authorized service center to have it checked. They can perform a more thorough diagnosis and determine if the issue is indeed hardware-related. If so, they can recommend the necessary repairs or replacements.
It is also important to note that attempting to fix the issue yourself may lead to further damage, and may even void your warranty. It's best to seek the assistance of a professional to ensure that your phone is properly taken care of. I hope this helps!