I have Plantronics BackBeat Fit 2100 headphones. They were working fine with my Moto X4 (Android 9, May 2019 update) until this afternoon, at which point the volume controls stopped working.
Before: the headphone volume and phone system volume were independent. The phone dictated the max volume the headphones could reach. If I set the volume on the headphones to their max, I would get a "Volume maximum" message played through the headphones, but not if I set the phone volume to its max.
After: the headphone volume and phone system volume are tied together. The volume controls on the headphones don't work at all. If I set the phone volume to maximum, I get the "Volume maximum" message played through the headphones.
I changed the developer option to disable "Absolute" volume, but this had no effect. What could have happened?