Hmmm. I have never seen that.
I set up Bluetooth between my iPhone and the car before I plugged in a USB cable and first tried CarPlay.
It was my impression that music was playing through CarPlay if the phone was plugged in. It does not, at least for me, seamlessly switch between Bluetooth audio and CarPlay. So if I unplug the phone but still want music from the phone I need to use the audio settings to select Bluetooth to get things going again.