Thanks for the reply.
I've cycled the router and iPhone to no avail. The HomePod requests do not show on the iPhone as personal requests.
The Home app on iPhone CAN create an alarm on the HomePod, but it doesn't sound at the appointed time, and the HomePod, when asked to list all alarms, says it can't find any.
The Home app on iPhone CANNOT create a timer on the HomePod — it says "Timers Unavailable / Timers can be viewed and managed only when this HomePod is on the same network as this iPhone." Both devices, however, are in fact on the same network.
Everything else is fine on the HomePod. I can make phone calls, control my house lights, check the weather, check the time, etc. It is only timers and alarms that have broken.
And Siri-set timers and alarms still work fine on my iPhone and iPad (which I'm having to fall back on.)
Since it's so persistent, I assume it isn't just an upstream server issue, and I'm hoping an OS update will fix this eventually. This is Version 16.4.
Thanks again.
Mark