Spotify over Bluetooth doesn't work at all in my 2015 Mini Cooper, but Google Play Music works fine. I've tried all the suggested troubleshooting tips in all the articles that mention Bluetooth, as far as I can tell without any changes, but Google Play Music keeps working fine. In the support forum this problem gets mentioned a lot and it seems to get deflected into topics like "metadata doesn't work", etc. But fundamentally there seems to be a problem in Spotify's Bluetooth implementation that produces erratic or zero Bluetooth functionality.
Examples of the problem:
Song is playing on your phone. You get into your car and turn it on. Car and phone recognize each other and Spotify thinks the music is playing (i.e. the progress bar is moving and the timers are ticking, but no sound.
Without changing anything in your car, switch to the Google Play Music app on your phone and start playing something. Sound comes out fine, and metadata and steering wheel controls work as expected.
Switch back to Spotify and try to play the song and you're back to the behavior described in the first bullet.
Attached are pictures of the two states: Google Play Music working, Spotify failing, and a screenshot of Spotify app failing to play on Bluetooth.