Music is taking over as audio source

Ever since I have gotten my iPhoneX, Music keeps taking over as the audio source when I reconnect to my car stereo. I use Pandora as my audio 99.9% of the time, and it is always what I listen to in the car. I can get in the car, and Music will start playing. I have to unlock my phone and tell Pandora to start playing again. Then if I get out of my car, and come back, Music takes over again and I have to again tell Pandora to start playing.

Previously, once I had Pandora set as the audio source (launch the app after a restart, etc.), then every time I get in my car, Pandora would be the source and start playing automatically.

My car stereo does not have CarPlay and I connect through a bluetooth interface. On the stereo, the source that is selected is "Bluetooth Device" and there is no native integration of Pandora within the stereo.