Descriptive flexfield default value is entered but not saved until you open the descriptive flexfile

Descriptive flexfield default value is entered but not saved until you open the descriptive flexfiled window or make changes to the Purchase order header.

 The descriptive flexfield PO Header has context-sensitive segment. Segment is set as required, default type is SQL Statement, the default value is: select case when vendor_name_alt is not null then vendor_name_alt else vendor_name end from po_headers_all p, po_vendors v where org_id in (322) and v.vendor_id=:PO_HEADERS.VENDOR_ID

 When I create the purchase order from Autocreate form, save Purchase order, then find this Purchase order in the PO_Header_all table, the according attribute has no value (null). But in the flexfield window I see that defaulted value is inserted. And when only I press OK in the descriptive flexfield window the default value is saved and I can see it in the PO HEader table.