IN Unit of Requisition Defaults Is Not Being Applied to Items Added to Cart?

Hello,

When creating a requisition, if I set up the Requisition Defaults by changing the Ship To field to something else and filling in the IN Unit field, then add items to the cart and checkout, the items have empty IN Unit fields in their Schedule Details. The BUSINESS_UNIT_IN field in the SHIPTO_TBL is Null.

However, the above does not occur if I set up the Requisition Defaults by only filling in the IN Unit field.

I would like to know why changing the Ship To field is preventing the IN Unit field from being applied to items I add to the cart, and how to make it so it is applied?