I've had this problem since before iOS11, so not sure if it's 11-specific, but I don't have or use Apple Music at all. I have manually synced all my playlists/music from iTunes over to my iPhone (6s), but I noticed that many songs in my playlist have that little 'download' icon, as if they are not actually on my phone. I only really started noticing this when a song would skip really quickly, but after this happening enough, or if I'm near a strong enough connection, it starts streaming a song (or so it seems), as it will start cutting in and out if I go into a poorer connection area. I should also say that my phone is nowhere near full, so it's not like it's syncing what it can, and leaving the rest of the playlist for streaming use. Is there a reason why this is happening at all? Is there a way to force my phone to physically have all of the songs that I tell it do (and not have some that stream)?