The Spotify app for Mac seems to take over bluetooth and do a few weird things.
If I am playing music through Spotify with my Bose QC35's and I have some other audio source coming from the computer (anything at all, an ad, notification, you tube video) then both streams play. If I pause Spotify to listen to the other source then I can't hear anything (any audio from any source) else from either my Macbook or any other device connected to my headphones. When I resume Spotify then Spotify continues to play normally.
If I close Spotify, I still can not listen to any other source of audio (still coming from my Macbook but say you tube for example) until I disconnect my headphones and re-connect.