judging by there the crack is, to where the bolt is. the bolt wouldn't of coursed that. it must of happened some other way. it is a very weird spot for a crack unless you had pressure from the sides. so yea i highly doubt its from the locking of the mech, maybe something else happend.
i suggest to get the crack welded up, put a bolt through the back of the neck. and should be good to go.
i suggest to get the crack welded up, put a bolt through the back of the neck. and should be good to go.