A ComboBox does not use the ButtonCell to render the selected item if the ComboBox is editable.
Instead, it renders the item via its toString()-method.

I expected ComboBoxes to behave the same in this regard, no matter the property's value - similar to editable TableViews, where a label is rendered until the user starts editing.
This label can be rendered using the ButtonCell.

For an example, please see
https://github.com/UrsKR/sandboxes/blob/9e6f678535d26c8560ce084286b561a9f68a0515/sandbox-javafx/src/main/java/bugs/combobox/RT37364/EditableComboboxDoesNotUseButtonCell.java