Reassigning a role on a CiviCase creates a new relationship but doesn't delete the old one

This is reproducible on the 4.7 demo site.

In 4.7, when managing a case, I assign a roIe to contact A, and then reassign the role to contact B. (When doing this I don't use the Add Role button but the button which has a pencil icon and whose hover text says 'reassign'.)

I expect that CiviCRM should delete the relationship with A and create one with B. However, in practice the relationship with A remains.

The function that is invoked is relationship() in CRM/Contact/Page/AJAX.php but the "cid" value is not set (this should be the Contact Id of the current person holding that role). So the existing relationship is not deleted.

I've marked this as Minor since there is a workaround: delete the original relationship.