9.2 Item Substitution Functionality

We are in the process of functional testing in 9.2 and are working throught the item substituion logic.  It works as expected when you substitute an item from the same vendor.  If you substitute from a different vendor, it does not work as expected.  It brings in the substitute item but does not update the supplier on the requisition with the new supplier that the substitute item will come from.  Can anyone explain this?