Rudder boost will not malfunction when both engines are operating as it is controlled by a rubber diaphragm and will have equal pressure on both sides. Any leak in the diaphragm will not have any affect until one engine fails. I once had the rudder boost operate the wrong way on an air test when I shut down the right engine because there was a small puncture in the diaphram but it was completely manageable and should not ever result in the situation shown in the video.
MM