I'm sorr you're having this issue. I can however say that it works just fine on my HTC M8.
I suspect it is your device/manufacturer's stock media player that isn't releasing the media player correctly. If you try to force stop / disable the stock media player I wonder if Spotify will start working as expected.
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