iMac with Apple TV and stereo HomePods. Will always play Ted talks. with sound from HomePods. But will not play HomePods when switching from Ted talks where it plays, to Channels DVR.
There is a known bug in tvOS where you have to reconnect to HomePod after waking the Apple TV from sleep. You can hold down the Play button on the home screen and deselect/reselect your HomePod.