Device: iPhone XS and iMac 4K
Operating System: iOS 12.1.2 and Mac OS 10.14.2
I came accross an odd bug. Here is what happens.
Playing Spotify on iPhone over AirPlay to an Apple TV. The Spotify application is open on the Mac connected to the iPhone which is playing the music. I invoke Siri on the Mac by clicking the menu bar button. When Siri starts listening, the music stops playing and when Siri is done, Spotify continues playing through the Mac speakers.
Remember, I was playing Spotify through the iPhone, not the Mac. Invoking Siri on the Mac shouldn't have done anything. The music should have continued playing over AirPlay.
If Spotify is not running on the Mac, this doesn't happen. And invoking Siri on the iPhone while Spotify is playing from it over AirPlay has not effect on Spotify playback. It does not pause and playback is not changed or interupted.