In the field properties dialog, each field has an ID number (leftmost column).  On the right side of the field properties dialog, I remember seeing a box that contained the field ID number for the selected field's parent field ... i.e. the field that the selected field appears under in the All Fields section of the Properties Pane, when "Tree View" is selected.  I can't find that Parent Field box anymore.  I guess that box has been removed from the Field Properties dialog, so that the parent field can no longer be set from that dialog?