Handoff to HomePod works sometimes, not others...

This is driving me mad. I have three stereo pairs and one single HomePod. ( I know, ridiculous. Guilty. But easier and frankly cheaper than running wires for in wall/ceiling speakers.)

Handoff to HomePod (seemingly?) works fine for me on all my multiple HomePods. But for my wife, who is in my iCloud family and was invited and accepted as an additional user through Home(Kit) App, handoff only works on *one* stereo pair of our HomePods and not any others. ??!?!

All HomePods are on 13.4.5. Both iPhones are on 13.5. Mine is an old SE, and hers is a XS. All the HomePods (properly paired) show up in here airplay list, and she can airplay to them manually just fine.

Handoff was previously working for her on a *different* stereo pair of HomePods (I didn't check all of them), than the one on which it is *now* working. Perhaps 13.4.5 had some deleterious impact?

I'm utterly flummoxed. This is worse than it not working universally. I've recycled her phone. I've rescinded and re-invited her through HomeKit. I've signed her out of iCloud and back in several times (which is scary). I've removed the HomePods and re-added them to my HomeKit Home. I've tried making a new HomeKit Home with a different name and adding the HomePods to that.

Nothing mattered, or at least mattered in a way that was I could detect. All the HomePods appear to work fine with my iPhone SE, and only one stereo pair works for my wife, the rest no joy.

Any advice?