I CAN DUPE THIS!
I've seen this issue since iOS14, and have today finally managed to reproduce it at will.
For *me* it's specific to my EPOS (pka Sennheiser) ADAPT 560 and ADAPT 660 Bluetooth headsets. It doesn't happen on my Poly UC8200.
All I need to do is receive a call on the phone when the headset is paired/active - or not even answer it - and after the call my Sounds & Haptics / Ringer and Alerts volume has jumped from next to zero to anywhere from 50% to 100%, ready to startle me the next time an alarm goes off.
Making a call doesn't do it. It seems to be tied to the incoming ringing volume.
I've separately engaged EPOS support with the details of the dupe.
My environment is an iPhone 8 running iOS 14.2. I'm also permanently paired with an old Apple watch and the phone is permanently in mute (or whatever the term is for when the side switch is activated).
My partner has just upgraded his iPhone [8] SE to 14.2 and with the same headset it doesn't manifest the issue.
I've previously reset all my settings at the behest of Apple support and that didn't do anything (other than give me a lot of settings to have to restore). The phone has NOT been reset to factory.
- G.