When I turn my screen off whilst listening to a track the track pauses after a few minutes. Why?

This is to do with your phone itself rather than the app. If you have Power Saving Mode activated then the app will stop playing after a few minutes. You will also find that the track then resumes again once you have unlocked your phone again etc etc. To stop this from happening simply turn off Power Saving Mode and listen to the whole track in peace without any interruptions.