Before my HTC One M7 was updated to Sense 6, I used to be able to kill the Spotify app in the Sense task manager without stopping my music from playing. Yet now it does. This is very annoying. Me and a lot of other people would be very satisfied if this bug were to be fixed.
Are you talking about the most recent update (last week or so)? Actually that's something introduced with Android 4.4.3 and is not a bug with the Spotify app... Some may call it a feature.
By closing the app most people expect the music to stop. I personally dn't use a Task Manager and there's a school of thought to say you shouldn't and should just let Android handle the min free values. I think it's just something Android introduced and you'll need to just change your habit to use the home button and allow Android to manage the processes, which it is very good at doing.
Please use the search feature before posting a problem/bug/idea. What's a Spotify Rock Star, and how do I become one? Using Spotify Since March 10th 2009 Twitter: @mechanimal82