Always use Warehouse Location from Inventory File Maintenance on Loading Sheet.
Change the sorting and display features in Print Invoice to use the Warehouse Location value from Inventory File Maintenance instead of the value stored with the invoice line item.
Reasoning: When certain System Option settings are enabled, adding an item to an invoice from Change Invoice will overwrite the Warehouse Location value stored with the line item with the value "ZZZZZ" to indicate that it is an "add-on" item this modification will prevent overwrite.
While this means that reprinting the Loading Sheet in the future might show different information than the original, there is currently no way to retrieve the Warehouse Location value that existed when the invoice was being created.