I have this problem for a couple of weeks now. When I save an album and then download songs on my phone, after some time they just disappear. Albums are still saved on other devices that I use for Spotify (like desktop app), but they disappear on my mobile and I have to save it for a second time (and also dowload it ofc).
However, everything looks fine with playlists - they are still downloaded.
I've already tried reinstalling the aplication a couple of times and also clearing it's data (both on my phone and also in the app).