I called Sage. first response was ""what are you trying to do?"" then ""ok let me understand what your trying to do?"" I get that. based on the responses here, it is rather unusual to update the Sales order from invoices.
After a remote connection and walking her through it all she finally understood. and is going to test and see if she can duplicate the issue.
in my opinion, it is broken. I THINK that I have done similar on the PO side with updating a PO from receipt of goods entry and it worked as expected. but I'm not quite sure if I did that to a purchase order or to inventory and I can't spend any more time today to test the PO side.
I'll keep you posted if I get any more concrete answers from Sage