When adding files to zones MC stops the currently playing zone.
I have 3 Zones set up, while playing an internet radio stream in "Zone 1" I used "Send To / Play (Zone 2) / Add"
upon which MC ceased playback on Zone 1.
I then re-started playback on Zone 1 and added additional tracks to Zone 2 using the same right-click menu commands, upon which MC ceased playback on Zone 1.
I have done no further testing. Need some done, just ask, same for additional info.