Link to existing bill: Linked attachment shows as the source document instead of the invoice that the purchase was created with

When linking documents (such as a purchase order) to an exiting bill, this document then shows up as the source document preview in the web browser version, instead of the invoice the ill was created with.
This is very frustrating when checking off invoices on payment day.
Does anyone know a way around this? 
Note that combining the documents into one prior to putting into the tray is too much hassle for our AP team.

Thanks!