Looks suspect to me. The location of the failure indicates there is not enough tread length to even engage half a nut, never mind a full nut. The engaged threads (only three) should have stripped if someone was tightening the bolt, yet there is no visible indications of galling or other signs of a nut being even partially threaded on to the nut.
One would expect the failure to be in the first one or two threads engaged in the nut, i.e., between the underside of the bolt head and the nut. One would not expect the last engaged thread to fail, i.e., the thread toward the end of the nut that is on the outside toward the free end of the bolt.
Something smells of a prank.
Al