I can't help but I'm getting something similar, here's hoping it's related.
Mountpoints are visible for me and I can select one but when I actually start downloading to use offline, my mounted drive volume doesn't change at all.
Weirdly enough, spotify seems to create the folder structure on the mounted drive - it just doesn't put anything in it. If I set the "save to" folder to be the internal default, I get files. If I then change that setting to the mounted drive, the next time I open Spotify, the files appear very briefly and are quickly wiped away.