Downloaded Spotify playlists vanishing

I've noticed in the last couple of weeks two, and on all occasions only the same two playlists have become "undownloaded" onto my S2. I've downloaded the both 4 times now, and on the last three occasions it took only a couple of minutes to donwload around 20 tracks at extreme quality.

This suggests that the files are not wiped off the phone, but somehow not registered as downloaded - I notice that if you undownload a file it doesnt appear to erase it, merely mark the phone memory as "free" and available space to be written to.

I'm wondering if this is a spotify problem, since no other playlists exhibit this behaviour, in addition, I have deleted more than the equivalent number of tracks in case it was a memory issue on the phone, though according to task manager I'm using around 2/3 or all availabale phone / sd card memory.