When the "Pending Reason" field is set, subsequent transitions will not clear that field.

If an issue has been approved (or otherwise moves out of the pending status), the field should be cleared automatically. Alternatively, provide a service-desk specific post function to clear this field so administrators can choose to opt-in or opt-out of this behaviour.

It does not make sense to have a "Pending Reason" stating why the issue is pending approval, when it might have already been approved (or the pending reason is no longer valid)