As for the reason of such behaviour is some SAP issue.
When I create NDF transaction I've got 'Fixing and cash settlement' area on the 'Structure' page.
But when I execute 'Fixing' this area disappears and I cannot specify fixing rate and forward rate separately. So without this rate difference system cannot calculate cash settlement amount. That is why flow 1031 is empty.
It seems that it's real SAP program error