I have this happen probably once a month, and its drives me nuts. It either says its playing and there is no sound, or it connects/disconnects constantly from CarPlay, and won't play anything. I'm still using an old iPhone 5, but everything is updated. I've narrowed it down to it only happens to me when I jump in the car, immediately plug in my phone, throw it in gear, and head down the road. If I wait 30 seconds or so until I am driving down the road to plug my phone in, it prevents it (at least so far). I'm guessing it has to do with the initialization of the infotainment system/backup camera/plugging the phone it all at once and something gets wonky. At least thats my best guess from the times its happened to me. When it does happen, the only fix I have found is to pull over, shut the car off, turn it back on, and reconnect.
I'm glad it doesn't happen often. I really like the CarPlay interface, but its a real PITA when its not working correctly.