Only way I could see it reverting back to FM was if your original source before leaving the vehicle didn't connect quickly enough for the system so it defaulted back. Could also be some settings in your phone/system from a bluetooth audio prospective. Just like the USB stick, that may just be how the system operates not necessarily a fault of yours or the vehicles. Would double check settings on your phone and Uconnect then if it still does it chalk it up to it being "normal".

 

If your key fob sets the seats and mirrors to your settings when you open the door, try resetting the memory with your audio on the setting you want it to be on. Worth a shot.