Galaxy s9 volume keys override car volume knob
 

I recently bought a Samsung Galaxy S9+. I also have an old Note 5. When the S9 is connected to my car via bluetooth and is playing music (usually Pandora), the volume knob on my car does not work and I have to adjust the volume using the S9's volume keys, which is annoying. This does not happen with the Note 5 is connected to the same car via bluetooth and playing music, i.e., I can adjust the volume with the knob on the car. Does anyone know how to fix this? The car is a 2017 Mercedes GLS450. Thanks in advance.

No solution to offer but just adding a data point here. We have a '16 GLE and I used to have a Samsung Galaxy S7. I stream music from apps regularly over bluetooth and with the S7, the volume knob on the dash (or steering wheel buttons) worked normally. A few moths ago, I upgraded to an S9 and have the same issue as the OP. The car's volume doesn't do anything but the buttons on the phone do. Sporadically, the knob on the dash will work to adjust the volume, but most of the time it does not.

Based on your description, the sound controls are different on the new S9 phone. What if you connect the phone cable for Android Auto? How does it behave?