When you create a requisition using the "Buy with a team" feature via Guided Buying - the Request On Behalf defaults the "Team Owner" as the On Behalf of. 

This is expected behavior and no matter which team member( team owner or contributor) is creating the PR the headers level information, such as On behalf of, Plant, company code, Purchasing Unit default from the Team owner.

The On Behalf of field is not editable.

SAP Ariba Buying