Always useful to call out the platform version you are running...
Aside from that... Have you tried using the Sync Now option in the General tab of the Hub Mesh page? I expect we should not need to use this, but may be worth a try...
Found the issue! I have the Home Assistant Device Bridge app running on the Hub 1. At some point in the past I had shared this device and it has hundreds of children representing devices that I use from Home Assistant.
I never actually used this device on the other hubs as I centralize most of my rules on Hub 1.
So I was in the process of setting up a test device and sharing it and noticed this device. So on a hunch I unshared it thinking the number of devices was overwhelming the mesh - or timing out or similar.
And sure enough all of the devices shared on Hub 1 miraculously appeared in the other hubs.