Hey! So I believe I finally figured it out! If shutting off battery optimization for spotify doesn't work, then you have to go to your settings... type in " face widgets " and uncheck music! Worked right away for me. Hopefully that solve your issue and everyone elses.
I don't understand though... Surely this thread is about the Face Widget being the part that's broken. I could have told you that your could change to the standard notification controls by turning it off weeks ago! That said, now that I've discovered that my headphone controls don't work properly this could be a good interim solution for that. The fact of the matter is that Amazon music works fine so it must be a Spotify issue. I think we just need to wait for them to fix it.
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.