Currently Business Central keeps upstream links between orders->receipts->invoices at the line level if you start with a downstream document and use the 'get-receipts' or 'get-orders' functions. There are many circumstances in which you want to use these functions but are retrieving a single order or receipt.

My view is that Business Central should be 'artificially intelligent' enough to recognise this circumstance and for simplicity copy the order or receipt reference at the header level - the same way it does if you use the post function directly from the originating document.

As a bonus having a field that indicates whether it is single or multiple sources for line items will help identify why the order number is missing in the header.

One hopes that as part of the refactoring they look to a single code base that acts a certain way depending on the data, rather than where a particular function was called from.