Hello! It's possible that your Redmi Note 8 Pro is experiencing a hardware issue that's causing the volume down button to malfunction. Here are some possible solutions that you can try:
1. Remove the phone case - Sometimes, phone cases can cause pressure on the buttons, which might result in them being triggered unintentionally. Try removing the phone case to see if the volume down issue persists.
2. Restart the device - Sometimes, simply restarting your phone can resolve software-related issues. Hold down the power button until the "Power off" option appears, and then select "Restart."
3. Clear cache partition - Clearing the cache partition can help resolve software-related issues. To do this, turn off your phone, and then press and hold the Volume Up and Power buttons until the Mi logo appears. Use the Volume buttons to navigate to "Recovery mode," and then press the power button to select it. Use the Volume buttons to navigate to "Wipe cache partition," and then press the power button to select it. Once the cache partition is cleared, use the Volume buttons to navigate to "Reboot system now," and then press the power button to select it.
4. Factory reset - If none of the above solutions work, you may need to perform a factory reset. This will erase all data on your phone, so make sure to back up any important files before doing this. To perform a factory reset, go to Settings > System > Reset > Erase all data.
If none of these solutions work, it's likely that your phone has a hardware issue that will require repair or replacement.