Got exactly the same Problem with OnePlus 7 Pro. Got the latest software and tried reinstalling Spotify. Nothing helped so far. I can skip one song and then the controls dont work anymore. My car ist an 2018 A-Class.
I have the same problem with my OnePlus 7 Pro. Also the track info isn't showing on the cars display. I had a Galaxy Note 8 before the OnePlus and it worked without issues. My car is a 2017 Alfa Romeo Giulietta.
The problem is the latest Spotify app is causing this issue. You notice how everything was working before all last year and all of a sudden this year with so many updates of the app now things aren't working. I had to downgrade my Spotify version. The software engineers changed something and now we have issues with the latest app.