I’m experiencing the same issue, Iphone 8, iOS 13.3, GMC Canyon. I have the ringer volume turned all the way up, and the change with buttons setting toggled off.
When I get in the vehicle and the phone connects, but the vehicle is playing some thing else (Sirius XM for example) and turn the vehicle off, nothing changes.
If I play something via Bluetooth, then turn off the car, the back on so that the phone auto-plays what was previously playing, when i get out of the car later, the volume drops by 60% or more. If I do the same thing multiple times (for instance running errands), at some point it will ringer volume will drop to 0%.
I’m missing important calls all over the place! Apple, please fix this! It shouldn’t matter what the ringer volume is set to - it should stay set, and not get changed by outside sources. the only time i’d consider that turning down the ringer volume would be okay is while connected (by usb cable) to the car, *and* using carplay. otherwise, stop messing with it!