I've searched on this topic and although there are a few threads I haven't come across an actual solution for it yet.
I've just purchased a new OnePlus 5 and decided to start giving Spotify a go over Google Play. When I connect via Bluetooth to my car Google Play and every other app works flawlessly except Spotify. It will connect, it looks like it's playing on the app but on the car I have no sound coming out and no track information. When intially starting if I switch to any other streaming app (G.Play) it will then reset and start playing and I can start playing. However if I then get out the car and get back in i can't get it to play at all.
Currently trialing premium spotify but if i can't find a solution i'll probably have to cancel.