i am getting the same result in my account - when i'm filtered to main line = false, no due dates appear. when i just display a column like line ID or account, the due date only appears on the lines that represent the header of the transaction. so yeah seems like a limitation of due date for some reason; but i would imagine if you're just sourcing from one body field to another, you don't even need a workflow; just set up a custom field to source from the native due date field