Requisition was created with supplier site field being blank, even though that is a required field

We have a requisition that was created using one of our smart forms. In this smart form, the supplier and supplier site fields are required, and typically the smart form will not allow the creation of a requisition if these fields are blank.

However, we identified that for a requisition that came through, one of the lines on that requisition was created without a supplier site. Consequently, a purchase order was not generated for this line and it is now sitting in our 'Process Requisitions' menu.

We would like to understand why this specific line was able to be created even though that field was left blank.