I was the same situation as yours, and no tips mentioned here couldn't solve the problem.
Finally I found the solution.
Just uninstall the latest spotify and install old one (such as Ver.8.4.9).
I had the issue of the spotify controls from the lock screen widget not working.
What resolved the issue for me was:
Settings > Lock Screen > Face Widgets > Turn off the music option
Spotify then worked for me straight away.At a guess I would say there might be some type of resource clash with the default samsung music which I don't use and spotify. So turning that off resolved the issue for me.
After trying every other solution on this thread, disabling FaceWidgets was the only that worked for me. My phone now uses Spotify's built-in lock screen controls and works fine.
Yes I have the same problem with YouTube when I am running I have to keep stopping and restart, also I have been very disappointed with the picture quality on my galaxy 10+ or maybe I have not gotten the use of it yet.
Having fixed mine it broke again so I rolled back to the normal Google Play Services etc. However, this weekend it seems to have fixed itself. It so happens that I also used my Headphones (with controls) on the plane so perhaps this is what fixed mine as well. Weird.
OK, so, after this broke yet again, I fixed it by putting my Headphones in...however, I don't think it actually started working until I'd clicked on the headphone controls the first time. Yesterday I'd tried simply plugging them in without success but today I used them properly to pause/skip forward etc.
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.