So I did a quick test with mine the other day....I was on the highway at 70mph, with the radio volume set on 11, (I was streaming music via Android Auto), and the music was plenty loud..
When I slowed to take an offramp down to about 35 or 40, I definitely heard the radio go down about 1/3rd in volume.. However the volume control was still set on 11 when I checked at the stop light.
So I just realized now I said I was "streaming" the music, and not listening to the FM radio....I wonder if that would make a difference???