For an aeroplane component to fail from fatigue, seems to indicate, that at least some calculations were faulty.
Not necessarily, the part is a machined forging and there is a possibility that during its processing it were damaged. A small inclusion in the forging, a mis-cut during the machining, a corrosion pit? Or the parts could have been damaged during installation of the actuator. Just because it failed does not mean the design was faulty.
I am sure the parts were give an complete examination including metallurigic analysis and the source of the failure has been identified.