I added a custom role for to allow only OrganizationDesigner capabilities. I could see functional positions but I could not modify or add them. What permissions I have to assign to role for adding and modifying functional position in organization designer.
I tryed everything bot noting works unless I give site addministrator privilages to the user.
I have just replicated the behavior you are describing in our training environments. This is not the expected behavior. I will immediately open a case with Customer Care, and I suggest you do the same.