I've have the center of a 428 "wagon wheel" flexplate shear, but never lost a bolt like that.
The tension on the bolt is expected to be much higher than the bending load that the fastener will see, with the friction between the two objects being clamped resisting the movement between the objects.
It seems that to have the fasteners shear, they must have loosened enough to relax the clamping load, then subjecting the fasteners to bending stresses and leading to failure.