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.
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