Suggested buyer mandatory not working when Requisition is submitted without saving

We have made Suggested Buyer mandatory through Visual Builder:

1- when we save the Requisition without populating the Suggested Buyer Field, a message is displayed stating that the suggested Buyer is mandatory as expected

2- When we submit the requisition for Approval directly without saving, the requisition is submitted with Suggested Buyer Field not populated.

Is this a bug, or there is a missing step in Visual Builder?

Thanks.