I've seen other similar posts about my problem but I don't think there's an effective solution yet. Every time I go offline, all my downloaded songs and albums go missing but the playlists i made and downloaded always remain. After opening the app again and clicking on songs or albums, it would be empty. Then, when i go online, the songs and albums i have already downloaded will reappear but are made unavailable offline. I really don't know what's causing this problem, I'm pretty sure my memory isn't full yet.
I am having a similar issue. I use Spotify on a Sony Xperia XZ with Android 7.1.1. My spotify is version 18.104.22.1689 arm V7. I should always have the most up to date version as mine is set to update automatically.
My trouble is that downloads simply disappear without rhyme or reason. I can listen to a downloaded album one day, and it's gone the next. I can download an album in the morning before work, and it's gone when I go to listen to it while travelling to work. My playlists don't seem to have this issue, it's only albums (or similarly complete artists if I've downloaded several albums from the same artist).
It's frustrating, I download music because I don't always have an internet connection for streaming (I also don't want to drain my data allowance by streaming, when I could download via WiFi)
As an example, the attached is a screenshot of one artist on my device. I switched the download switch on just two days ago, downloaded her music via WiFi, and it has mostly gone today (the only tracks still downloaded are in playlists)
I'm not lacking in space on my phone, showing over 12Gb free on my SD card which is where Spotify is storing my music.
This has happened twice for me. First time my data went over by crazy since the songs started downloading again when not on wi-fi. Extremely frustrating and irresponsible of Spotify to allow this to happen when it's a reason people are paying for premium service. It just happened to me again and only a few random songs on my large Playlist are available offline. No solution has been posted for this issue yet so please take this seriously.