Short story: I have Spotify on my home pc, work laptop and Android phone (I'm using it with Android Auto).
If I'm on my main PC and I click the + button on a song, I can find it in the "Tracks" section on my library, on any device.
However, if I do it from my phone/work laptop, the track won't be added to my main's pc library.
Quick example: I click + on a track from my work laptop, I leave the office and I can keep on listening to it in my car using Android Auto. Once I get home I open the "Tracks" section and boom, the track is not there anymore. If I look on my phone I can still see it in that playlist, but as soon as I do something on my desktop I lose the track from my phone as well.
This is REALLY annoying, as I lost a lot of newly discovered music before finding out this sync issue.