Why is PO distribution DFF mandatory in Redwood but not in Classic PO creation?

Summary:

We have one non mandatory DFF on our PO distribution line. It is not a required DFF and is used for particular POs (not all). Having switched on Redwood PO creation, I am finding that I am being forced to enter a value in this DFF. I have tried using Visual Builder Express to see if I can do something to stop this behaviour but there is no 'More Details' section to be accessed.

Content (please ensure you mask any confidential information):

Try to create a PO in Redwood UI, but cannot submit the PO as a non mandatory DFF in Classic, is mandating I enter a value in Redwood UI. This will make it impossible for our users to create Redwood POs.