Many years here as a metallurgist doing all sorts of materials engineering work, in fact too many to count at the moment. Cannot tell if bolt is hardened, i.e. grade 8 for example by the photo. Photo does suggest a fatigue crack which could be caused by a number of things. Overtorqued, but not to the point of failure, increases mean load on bolt, leading to shorter life allowable vibratory life. Bolt clamp insufficient, i.e. undertorqued due to possibly the 2 mating pieces not being fully home or together for some reason. Just a couple of thoughts off the top of my head, not including defective bolt, possible though rare I would think. One final thought is bolt the grade it is supposed to be. Good luck. Mike.