Zetadocs Express, Get Shipment Lines in Sales Invoice doesn't include the files added to the posted Sales Shipments

I have the following question regarding Zetadocs Express, working on NAV 2013R2. If I have two Sales orders and add a document to both of them, let’s say Doc 1 to Sales Order 1, and Doc 2 to Sales Order 2. I post the sales orders and choose in both cases “Ship” (not Ship & Invoice). In the posted Sales Shipment, I can find Doc 1 and Doc 2.

Now I’m going to create an invoice and use the function “Get Shipment Lines…”. I select both shipments and insert them in my invoice. This goes without a problem, except that Doc 1 and Doc 2 are not copied to the invoice. Is this a bug?