This is working as defined, but I admit it's a bit confusing. There is one more step you need to do. Assign the middle mouse button to open the popup group, instead of the popup toolbar.
In Options - Mouse, Middle mouse button, Popup this Toolbar:, change the assignment from "default.Popup" in the Toolbars section to "default.Popup" in the Group section at the top of the list.
Once that is set, any more tabs you add to the group will show up. The rub was there was no default.popup group until you modified the default.popup toolbar.
Every time i upgarde Rhino, with a new beta version.. i just loose the popup modified. I have tried saving..and in that case you have to reopen the old toolbar file. This process is still not streamlined.
This is the 5th time that i am recreating the popup toolbar, and that is frustrating because you dont remember the old sequence and you keep looking for the icons at the wrong places in the toolbar (I add almost everything that i need in this toolbar - so it is like 60+ tools)
So can i just save the popup tool bar and migrate to another system without actually copying the whole toolbar?