Sound source keeps switching

Sound like it is your phone or maybe the car's Bluetooth receiver that is not successfully re-connecting every time you start the car. At least in my PP, all three iterations 2017 PP Premium, 2020 PP LE, and current 2021 PP Limited which is the same year model and trim as yours, if Bluetooth is selected as a source, it will re-connect and start playing the music right from where it was shut down. If you continue to listen to the same music source after you shut off the car, I suppose, then it will not start where it was disconnected from the car.

The "Source" stays the same every time I start the car unless that source is no longer available or recognizable by the car.

The automatic Bluetooth re-connect in my PP is so good, such that it re-connects playing the music moment I start the car, sometimes even if the phone is not inside of the car. This sometimes annoys me, I share my car with my wife. When she drives the car and has been listening to music via Bluetooth on her phone. The next time when I get in the car, music from my wife's phone starts playing even though she is not in the car, thus her phone is not in the car either. The music stops when Bluetooth drops after I drive off our driveway.