I'm frustrated. There have been a few threads on this subject and no reply from Spotify support.
When you get in your car and want to play ONLY your downloaded music, you can put Spotify in offline mode. Great.
BUT, when you're driving and can't touch your phone so would like Google Assistant to play a track for you, or switch playlists, etc -- NOPE. Doesn't work.
I can disable offline mode, but Google Assistant + spotify don't always play the correct track or album when I request it, and I end up streaming rather than playing from my downloaded content.
This seems like a very basic and useful bit of functionality to have work well while driving.