Funny, I just had the same issue recur today and was going to write in yet again about it. I went and got on the Tube for my usual 45-minute ride, which is only bearable with music, and discovered that of the 10 playlists I have set to "Play Offline", ALL of them had zero songs on them, meaning that I had nothing to listen to yet again. Frustrating.
Here's my theory: if your phone starts to get close to full storage capacity (which mine did last week), then for some reason when Spotify goes to try to sync and can't add an additional song, it freaks out and deletes everything it's already stored.
If that's the case, instead of me having to pay £500 for a bigger phone it'd be great if Spotify could just *not delete local songs unless I explicitly tell it to*.
Same thing here. I open Spotify and see that half the songs that I saved offline are undownloaded. Furthermore, songs that ARE offline (have the green arrow), don't show show up as such when I'm browsing by artist.
For example, I download the entire Rust In Peace by Megadeth. When I scroll through my artists, Megadeth has no green arrow next to it, but the individual songs do.
Well I have a Xperia Z5 and this happens to me every day and I just use this phone + Spotify since 3 days.. Also it sometimes freezes when I click the connected devices button to change the music to my PC. Sometimes it happens few times and work after restarting a few times. Hilarious..