Perhaps you are attempting to shut the iPhone OFF incorrectly. You do not have to hold any volume buttons to turn your iPhone OFF. What you can do is Press and Hold the Right Side Button until you see the Slide to Turn OFF option.
The other way to shut down your iPhone is to open Settings ➔ General ➔ Shut Down (Located all the way down).
If, for any reason, none of the above methods work, then Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:
- Press and quickly release Volume UP button
- Press and quickly release Volume DOWN button
- Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF).
Axel F.