FYI, it's gotta be an Apple bug. Today, when driving to lunch, my radio was on FM when I exited the car in the morning. Get in at lunch time, it switches to Car Play. I used steering wheel controls, navigated to FM, radio started playing, 15 seconds later, CarPlay as the source again, change to FM, 15s later, back to CarPlay. Use infotainment screen to switch source to FM, 15s later, back to CarPlay.
Surfing web, found several articles/references to this issue, not only on GM vehicles:
discussions.apple.com
https://discussions.apple.com/thread/252349057 (this one implies maybe Waze is to blame, will try killing Waze on my phone, although I didn't think it was running)
There are more...anyway...will try killing Waze and report back. Good luck all.