Well that was a rabbit hole. Turns out I was wrong with my assumption.
There is a problem adding nodes to a combobox as when an item with a Node is selected, the standard cell factory removes the Node from the list view and adds it to the button area. Which is why the text disappears. It should return it to the list view once another item is selected but it does not re-appear.
A bit more digging and I found a solution. You need create the items as standard text items (no nodes) and add a style sheet that contains:
.combo-box .list-cell {
-fx-alignment: center;
}
If you only want to center text on specific comboboxes, then you need to add a styleclass to the combobox and change the CSS to something like:
.cmb-center .list-cell {
-fx-alignment: center;
}
assuming cmb-center is the class you give it.
The attached project demonstrates with a standard ComboBox and B4xCombobox.