Subinventory gets default in backorder line

Subinventory gets default in Backorder lines-

I have a scenario for IR-ISO process. There is only one item in IR-ISO transaction and less quantity available in stores for the requested item. After the line is fulfilled the remaining quantities get backordered and split in sales order. However the default subinventory in the backorder line is still coming which is same as from which the first line gets picked. Now when the material is available again in different store backorder line doesn't get picked due to this sub-inventory defaulting

Example: steps

a) Ir - > ISO

b) Item 1 is raised for 10 Each (no subinventory defaults in the sales order)