I had this issue about a month ago and then it stopped. Spotify seems to randomly delete the music I've downloaded onto my phone. I've done everything short of a full factory reset of my phone. I've cleared off the music and redownloaded it, reinstalled the app, looked for settings on my phone that may be deleting files, but there's nothing.
It doesn't matter how frequently I listen to the music and doesn't seem to have a pattern. It just randomly deletes music.
I've opened Spotify before I leave to go out and all the music is there, but the next time I'll open it, everything will be gone
I have an LG G3 D852 running Android 5.0.1
I've never seen any posts on this issue and I'm desperate because this pisses me off.