I too have seen conflicts between CarPlay and the radio. My car's a Hyundai.

I mostly listen to the radio, with an occasional trip where I instead listen to a podcast or music via CarPlay.

What I've found greatly helps is when I finish a trip where I've used CarPlay after I park but before turning the car off I open my phone, open control center, tap the now playing widget, tap the symbol on that for output selection, and make sure it is set to iPhone Speaker. I then hit the Media button in the car and select FM. That starts the radio playing. Then I stop the radio and shut off the car.

Next time I use the car CarPlay connects normally but does NOT take over the media playback. If I hit the play button in the car it starts playing the radio.

This works fine for me because as I said I mostly listen to radio. It is not that big of a deal the once a week or so that I listen via CarPlay to do those extra steps at the end of a trip.

It would probably be a lot more annoying if I was frequently switching between radio and CarPlay.

In either case I would definitely like a setting somewhere that makes it so the play button in the car plays whatever source was playing the last time you turned off playback.