Mine used to do that as well but I went into device maintenance battery and made sure that the app wasn't set to be put to sleep after the screen shuts off. That worked for me.
I would recommend checking your Sleeping application list to ensure that Spotify is not listed as one.
Sleeping applications are not allowed to run in the background when the screen timeout is triggered.
I also recommend checking to see if the application is being optimized for performance.
Here is a troubleshooting article that can assist further: http://www.samsung.com/us/support/troubleshooting/TSG01200668/
Hope this message finds you well!