I have an iPad in Room A, set up for "Hey Siri" and almost always plugged in. I also have HomePod two rooms away, in Room C.
Whenever I invoke "Hey Siri" in Room A, the iPad lights up, Siri starts up then stops, then the session is transferred to the HomePod and Siri gives me the prompt for my question from Room C, and answers from there too. It would be great since HomePod has better microphones and speakers, but since it's far away, Siri has trouble hearing my questions clearly, and I have trouble hearing the answers clearly.
I even tried doing "Hey Siri" quietly to the iPad with almost zero chance HomePod could pick it up. Same behavior.
All devices are current firmware as of today (iOS 12 and equivalent). HomePod is a (typically standby) HomeKit hub (an AppleTV is usually the Connected hub), and the iPad is not a HomeKit hub, if that makes any difference. Wi-Fi and Bluetooth are both turned On for the iPad, and the iPad, HomePod, and AppleTV are all on the. same local network.
Is there a way to get the closer device to handle the Siri session directly?