This happens every week or two. I never stream music. I always add it to my Music library then immediately make available for playback offline. However I'll go out into town. Get out my iPhone and headphones and go to play a song and I'll get the error message that says I cannot play the track as I'm not connected (I deny Apple Music data use). It appears to delete my music from the device for no reason. I download all the tracks via wifi. But then randomly when I'm out they're not available. Does any one have a solution?