Just trying to think a bit.
If a bearing fails, or a gear starts to fail, making bigger parts come loose thats to big to hit the magnetic plug/chip detection system (like parts of cogs or so)...if these parts jam between the gears...could that cause the epicyclic gear to brake ? ..and/or possibly make the MGB suffer a sudden decrease in speed and/or sudden stop ?
My thought is that it is not sure that the origin of the fault was in the epicyclic ? If so, there shouldnt have helped if HUMS could detect cracks in the epicyclic gear ?