(On WP8.1)
For some reason a trial version of a soundtrack plays instead of the full version I bought when I tell Cortana to play that soundtrack. I checked in the Xbox music app to see if I had any soundtracks that only played if they are streamed and there weren't any. Next I tried deleting all the music from my phone and syncing it all back through USB and W8.1 File Explorer, then deleting any streaming soundtrack duplicates from my phone that showed up but it still happens.
For some reason a trial version of a soundtrack plays instead of the full version I bought when I tell Cortana to play that soundtrack. I checked in the Xbox music app to see if I had any soundtracks that only played if they are streamed and there weren't any. Next I tried deleting all the music from my phone and syncing it all back through USB and W8.1 File Explorer, then deleting any streaming soundtrack duplicates from my phone that showed up but it still happens.