I'm using Spotify premium on android (s7). I have a lot of albums (100s) in my library. I tend to download all my albums so i can listen to them any time.
According to the article below my downloaded albums should persist. But every now and then, albums which were once downloaded no longer are and i have to re-download them. This is particularly annoying when each album has to be downloaded individually...
If you cleared the spotify app cache then any downloads will have been removed. Wiping the system cache partition would have removed temporary system files and data and shouldn't affect your spotify downloads.
I only ever clear the system cache using ccleaner. I've just checked the app info and under storage there are two options "clear data" and "clear cache" - so as long as i don't clear the data my downloaded albums should persist. As i never do this i don't understand why albums that are downloaded suddenly stopping being downloaded.
As I said above, clearing the spotify cache will remove all downloaded music. If you use Ccleaner, you should set an exception so it doesn't touch the Spotify cache or you will have problems such as your downloads being removed.