Redwood Process Requisitions -how to hide the Sourcing Method field

On the Redwood Process Requisitions page, in the Sourcing Details region of a staged document, we need to hide the Sourcing Method field. The field isn't relevant for our buyers and creates confusion.

We tried two approaches, neither of which worked:

Visual Builder Studio (Express mode) - the SourcingMethod field appears in Configure Fields and Regions, but its Hidden property is locked and can't be overridden by an extension rule.
Security - the field still displays for users who don't hold the Create Supplier Negotiation privilege, and after removing Manage negotiations access in Manage Procurement Agents.

Could you advise how we can hide this field? If it isn't possible in the current release, please confirm that, and let us know whether the locked Hidden property is intentional.