I use a “control” database for my drop down boxes.The database has a “defaultFieldName” record containing only one value that I use in the Default Value of the field.
I also have a “Field Values” record that contains the list for the drop down.
The default is selected from the Field Values list and doesn’t allow for items not in the list.
The formulas work in all other field types that allow formulas for selection - just not ComboBox