Galaxy s8 and 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 my 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? I'm not sure it matters, but the car is a 2017 Mercedes GLS450. Thanks in advance.

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