There's still something about CarPlay that does that kind of thing but not tied to a specific song. I'll be in my car listening to the car's radio, with my phone on its mount but inactive and all is well. The phone connected wirelessly to CarPlay when I started the car, but I'm not using any CarPlay features on this trip.

Then I need to activate the phone (say I'm pulling into the McDonalds parking lot and need to look at the McD app to get my drive thru pickup code for my order), so I tap it and swipe up...and the car switches to playing in whatever app I last used in CarPlay such as Podcasts or Spotify.

If I hit the media button to bring up the car's media selection screen and switch it back to radio that plays for a few seconds and then it switches back to CarPlay.

If on the phone I go to the now playing thingy in control center and hit the gizmo for selecting where to play I can explicitly switch it from CarPlay to iPhone Speaker and then it stops messing with the radio.

Some Googling and some asking LLMs turned up that a lot of people have problems with CarPlay overriding the car's entertainment system and apparently nobody has a fully satisfactory way to deal with it. Some people have addressed it by using Shortcuts automation to pause playback whenever the phone wakes up. They still get interrupted, but at least it doesn't keep interrupting.