Mmmm, that's an odd one I can not say I have seen or heard of that before, are you sure the nut has sheared off ? or has it just come loose and dropped off, there would have to have been a lost of force to shear a bolt that size and I would of expected some other damage or trace if that had happened unless the bolt was faulty with a hair line crack for example, I think I would be inclined to replace the bolt and put the nut on with lock tight.
sorry that is the best I can come up with at present.