I called AppleCare this morning and was lucky to end up talking to a support rep that was very familiar with HomePod. In fact, he said he was working from home and had two HomePods himself.
While he listened to me giving Siri voice commands to the HomePods, it was easy to replicate the issue I described above. It happened the first time I tried it. But then he suggested trying it again. To my surprise it worked correctly. In all, he spent about 20 minutes with me on the phone while I tried various commands:
"Hey Siri, raise the volume in the kitchen to 50%."
"Hey Siri, play Hotel California by the Eagles in the bedroom."
"Hey Siri, what is the volume level in the bedroom?"
"Hey Siri, what time is it?"
"Hey Siri, what is the weather like today?"
Etc, etc.
The informational commands (time, weather, etc.) all worked as they should. However, at one point I was in the bedroom and, after Siri responded with the information, she didn't resume the music in the bedroom (but music kept playing in the kitchen).
In all, about 1 in every 5 or 6 commands something went wrong. In fact, at one point, Siri literally responded "I'm sorry, but something has gone wrong."
So, clearly, AirPlay 2 *is* designed to work like the the Alexa/SONOS combo with regard to controlling music in multiple rooms. But, just as clearly, there are bugs. The Apple rep even agreed that there are bugs. He said he was typing up extensive notes and would submit them to the appropriate software engineering team.
He also suggested that server load, particularly yesterday, might be a factor.
I'm a bit more optimistic after my conversation with the Apple rep. Since I purchased my second HomePod at Costco I have 90 days to return it. We'll see how it goes.
Mark