I've been having this issue randomly with my Sasmung Galaxy S4 mini. I use spotify on my laptop, tablet and phone. My tablet and phone have different albums saved for offline listening. I also sign in to spotify on school computers.
The offline library on my phone keeps getting wiped out. At first I had a few albums saved for offline listening and then I noticed a few weeks ago that they were gone. I most definitely did not delete them myself, nor had i reinstalled the application or even logged out. Those were saved to the phones storage. I recently got an SD card and was downloading my library for offline listening onto it. The download was taking hours so I only checked it intermittently. I saw it was at 85% completion but when I checked it again it was at 33% with songs that i had visually seen to be downloaded, not downloaded anymore.
My download setting is set to Extreme Quality. My internet is high speed and was uninterupted throughout. My phone did not shut off or lose signal. I don't really need a solution to this as I can just redownload the songs but I thought I should at least complain about it. Hopefully it gets fixed and makes spotify a more enjoyable app.