I had this issue recently - when I first paired my HomePods it worked fine, but I had to reset them recently and afterward Siri came from the other HomePod, which wasn't the one I wanted.
The fix seems to be simple: Siri will respond from the last HomePod you forcefully used it from. To force which HomePod Siri activates from, hold your finger gently on the screen of the HomePod you want to use, and Siri will come up (assuming you haven't changed this default behavior). I don't know if you need to issue it a command or if simply holding down at one time is good enough, but I issued it a command, and ever since then Siri has responded from that HomePod.
Truth be told, I haven't tried switching it back; I suppose that would be the definitive way to ensure that this is reproducible. But it's a bit more simple than moving the HomePods around. I am surprised Apple didn't include an option for this in software, as it isn't overly intuitive.