Okay, in another forum, I found this:
If your phone's security is set to auto-lock some interval after screen off (let's say 5 minutes), then during that interval several controls apparently do not work, including:
- Adjust the volume using the hardware volume rocker
- Cannot adjust the volume using any sort of headphone control
- Cannot pause or start the music
One workaround would be to set security to instantly lock after screen off and when pushing the power button (which is my setup and explains why I did not notice the issue when I tested).