I have the similar problem, but not regarding to HomePod. I can’t find any airplay speakers in the audio source list except the tv speaker. But before updating to the beta version, it’s fine. So I also reckon that should be a kind of bug of the beta version. Waiting for the next version...🤞

I knew that I can’t set the airplay speakers as default and have to choose them as audio output before I play any video on Apple TV, but at least I can find those speakers in the list, then choose. But now, in the list, I only can see the TV speaker, no others. So this is the change of the tvOS beta version. This is what I mean. For the other Apple decices, like iPhone and iPad, I still can find and choose airplay speakers.

yes, I have a similar issue. I just updated tvOS to the latest beta version 14.2, now I can't find my airplay speakers in the audio output option except the tv speaker. Before that I can choose any of all my airplay speakers. So I think it should be the tvOS issue. hope apple will be aware of that. btw, my airplay speakers are from ikea x Sonos.