This is not so much a bug but rather how the BT implementation is designed (i.e. it's normal behavior). It's done so the audio is not abruptly switched over or disconnected. Whether it's a good approach or not is another story.
What I usually do is switch the audio to the speakerphone before I shut the car off. Once the audio is not streaming through the head unit, the BT will disconnect when the car is off. You can also disconnect the phone from BT via the phone screen but that takes a few seconds to do.
I would suggest that my BTs don't auto switch. Mine stops and I have to restart after the new WAN connection is made. My apps upchuck when I leave one wi-fi net and it auto switches to another, including switching from wi-fi to LTE. None of these switch smoothly. It probably is an app limitation, coupled with the BT implementation, coupled with issues with wi-fi.
But, as I said, I don't want to waste time trying to troubleshoot it, I just back out and restart.
I had wasted much time troubleshooting the favorites, downloading and uploading POIs and contacts, with GM that I finally gave up. The USB system does not work as the manual says it should, but until they start paying me to identify what they did wrong, I will go on to fight other battles.