Because you're not supposed to need to set this preference, there are a lot of networking variables that could result in one device being promoted to primary in the Home hub list at any given time. That doesn't mean it is immune to bugs with other devices, like whatever bugs are afflicting your OG HomePods. Those need to be fixed. The idea behind multiple Home hubs is high availability that persists through common networking and routing issues. Letting you just choose one is not a better overall solution, even if it serves as a workaround for your immediate problem.
If there are a lot of networking variables why isn't apple dynamically setting the primary hub intelligently then? A lot of us here have to restart home kit hub devices in a certain order to get our Apple TVs to become the "primary hub". That implies it's not basing it on network variables but order on the network.