I haven't had that happen with the radio, but I've had something similar with the bluetooth audio. I play my music from mp3s and the app Poweramp on my phone. When I first started doing that, I always started and stopped playing by first going to Bluetooth audio on the car's main screen, then pushing play or stop on the phone app. Then one day I needed to stop it playing while I was driving because of traffic conditions so I hit the big round play/stop button on the dash. After I started playing music again, I assumed it would go back to normal, but instead it would always start up the music from my phone again as soon as I got in the car, even if I hadn't started the app and the screen was locked. Then if I turned it off using the dash button, it would turn off the phone, but I couldn't turn it back on to play unless I pushed both the phone and dash buttons. Now it seems to be more or less back to the way it was at first, but I've come to like using the dash button as it's safer not having to fiddle with my phone.If you see the volume slider move with the dial below the unit -- have you tried just pushing the "play" button inside the dial, or the right-facing arrow button on the display itself?
On a related note, I'd wondered why the radio wouldn't come back on each time I started; I'd have to hit the 'play'. Now, it seems like it won't stay OFF! Even if I hit 'pause' before getting out, it seems to come on all the time when I get in. An odd thing, I hope they fix it at some point.