Just like this topic I have sort of the same problem with pairing two devices: https://community.bose.com/t5/Headphones-Archive/QC-35-II-pairing-2-devices/td-p/107555/page/2
I pair my Macbook Pro and Samsung Galaxy A8. When they are both connected and I start playing something at my macbook I need to stop the connection from the macbook before I can hear something from my Samsung. So the multiple connections are nothing worth for me at the moment.
And yes I already cleared the bluetooth list connected to the devices again and did a reset of the headphone. And I've the newest firmware.
This post has got a dirty quick fix for the problem: https://community.bose.com/t5/Headphones-Archive/QC35-ii-Two-Bluetooth-devices-problem-only-one-of-t...
Spotify was active in the background, not playing anything, but this blocked to switch back to the sound of my phone. When I completely close Spotify on my Mac it will switch back.
This seems a problem for over two years and is not only for Spotify, but also other music apps on a Mac.
Why doesn't fix Bose this?