I have 1670 songs in a playlist (the old favourites-playlist) and I've been having a problem where most of the songs go from being downloaded and available to greyed out and unavailable for no reason.

I first make them available offline, wait until they are all downloaded over wifi and then I go offline. The songs are all there for a few hours but then suddenly most of them are gone and when I go online again spotify wants to download bout 1100 songs, that are already downloaded... 

I cleared cache and data. 

I uninstalled the app. 

Reinstalled. 

Downloaded all songs, all of them were downloaded and had the green arrow. 

Went offline, the songs were still there. 

Restarted the phone, the songs were still there. 

Next time I checked, a few hours later, about 1100 songs were greyed out again... I had done nothing with the phone, it just happens. 

Xperia Z3 Compact. 

Spotify version 3.1.0.1117 (it's been like this with the latest updates, maybe for about 2-3 months? 

Quality: Extreme (and I'm not interested in trying anything below that) 

Keeping the songs on the SD-card, internal memory too small.