All are running 16.4, as are all my hubs. I updated to the new architecture first time round before it was pulled, and I've not been given any option to re-upgrade it since installing 16.4.
All family members are running 16.4 beta? wild.
It's ironic, but personally I've found HomeKit to work much better since I installed the original architecture update - much quicker responses and virtually no failures. But none of that is much use if none of my family can use it.
The old architecture had several modes, including reduced capability operation without any home hub at all. In this mode, you are inviting people to control the individual devices in your home directly.
You could also have multiple homes on your account, while homes themselves I believe did not have their own independent 'identity' in Apple's cloud. So I've suspected individual people get their own 'copy' of the home, and get provisioned to control each device.
A lot of speculation because I haven't gone diving into old HomeKit hardware developer documentation to extract architecture details.
Matter requires a hub (well, one or more 'admins' in Matter sense ). The new home architecture now also requires a hub.
So my suspicion is that it is a conflict between these two world views on upgrade - the new architecture has a 'true' view of the world in the sense of apple hubs acting as an admin, but third parties being invited not knowing if are being invited to the 'new' home hub architecture or setting up their own 'view' of a home under the old architecture.
Apple had a support document briefly before the update was pulled where they recommended going in on the devices of those you invited and making sure there were no empty homes (under the old architecture, presumably) blocking the invite. They also advised to reboot all home hub devices (Apple TV and HomePods) in the home.
Finally, they said if the invited person is part of a home with accessories, Apple Support would have to be contacted to fix the situation.
I believe that support article is
now removed, as the option to upgrade was removed while the current betas are working toward a software fix.