Mandatory Supplier field bypassed on Purchase Requisition after removing Contract Purchase Agreement

We've identified what appears to be a validation gap in Self Service Procurement that allows a Purchase Requisition to be submitted for approval with the Supplier field blank, even though Supplier is configured as a mandatory field and normally blocks submission.

Steps to Reproduce:

1. Create a Purchase Requisition using a Contract Purchase Agreement (CPA). Selecting the CPA auto-populates the Supplier field and locks it from manual edit.
2. Add the line to cart.
3. Edit the requisition and remove the Contract Purchase Agreement number from the line. Instead of tabbing out of the field, go directly to Save and Close. (Note: tabbing out of the field after removing the CPA does correctly clear the Supplier field and blocks save — the issue only occurs when you save without tabbing out.)