Multiple devices trying to stream music

So, this is something new (perhaps starting with iOS 14.2, maybe earlier). I go out for a walk, and stream music (via my Apple Music subscription) through my iPhone 12. Meanwhile, my wife is at home, and asks Siri to play Christmas tunes on our HomePod.

Immediately, the two devices start having a conflict. The music simply stops playing on my iPhone. I raise my iPhone, and see a text box with "More than one device is trying to play music." I press the play button on my iPhone, and that causes the HomePod back in the kitchen to stop playing Christmas carols! My wife starts the music again, and the music stream is interrupted on my iPhone, and on and on it goes...

What the heck? I have family sharing turned on, and the HomePod has my Apple ID set as the Primary User in the Home app on my iPhone. How is this a conflict? And what the heck is a "HomePod Account"?