Spotify wants all the music in the world available for streaming.
When an album is removed from the library, it's always a decision of the label, or of the right owners or of the artist. Sometimes, It could be also a licensing problem.
In the case of The Magnetic Fields 50 song memoir, as the album has not been totally removed but just replaced by a shorter version, we can imagine a label decision.
Something to keep in mind is that Spotify never takes the decision to remove an album.
The best way to know exactly what happened, is to try to contact the label.
Thanks for your understanding 🙂