Right, I've sorted mine. Did the same as a few weeks back. Clean uninstall of Spotify (force stop, clear cache, clear data). Restart phone. Install Spotify. Before launching app, change power optimisation to off. Launch app, log in, sorted! Interestingly, you can't swipe the app away from the notification shade when music playing is...I'd read someone else say they thought that doing this was the way to break the functionality so hopefully this time it will stay working.