E.g. frmX includes subform in layout.
Column references subform/ref.
Database refresh yields None in view column.
Change column to return plominoDocument.ref (instead of field reference), and column is fixed.
I do not reproduce this problem.
Moreover, nothing distinguish a subform and a form, so there is not reason the column behaves differently here.
I guess there is something wrong somewhere else in your db (possible name conflict?)