Basically as mentioned in title. Not sure what actually triggers the files to go "missing" (I use inverted commas because its not really missing - read below), but it usually happens after the phone restarts (on its own when battery is depleted, or when I restart on my own).
I've tried the following remedies after reading around and searching other sites:
1. Reinstalled app. Still happens when phone is restarted.
2. Wiping cache & data > Restart phone > Log in > Redownloaded > Restart phone > Offline songs gone again.
3. Wiping cache & data > Removing folder in SD card (SDCard\Android\data\com.spotify.songs) > Restart phone > Redownloaded > Restart phone > Offline songs gone again.
4. Ensured that I do not have more than 3 accounts in sync at my offline devices settings.
5. SD Card has more than enough space -- about 20gb of available space.
It is only about 5 minutes ago I discovered something new -- after learning that the downloaded offline songs are gone again, I went into the folder in my SD card (SDCard\Android\data\com.spotify.songs) and realised that the songs are still there! Spotify apparently doesn't seem to be able to detect when my phone restarts.
In another thread I read that a user had recently upgraded his SD card to a bigger capacity before facing this problem. And then it struck me too that I had no problems before upgrading too! Seems like I've only started facing this problem after upgrading. But I've removed the folder and reinstalled to no avail.
Spotify version: 126.96.36.1994
Spotify account: Premium
Device model: Samsung Note 3 (SM-N9005)
Android version: 5.0
SD Card: SanDisk SL64G (64gb)
Telco. provider: StarHub