I have the same problem. If I'm listening to the radio or Sirius, and I receive a text, it switches to Source:Carplay and Apple Music to whatever I happened to be listening to previously on the phone. I also have the same problem as you, heffneil, after a call.
Also, every time i get in the truck, the radio says 'Loading...." forever until I hit the up or down channel button behind the steering wheel and only then will it start playing.
My dealer has no idea, GM customer service has no idea, and OnStar has no idea.