It was an inspection failure from SpaceX's perspective. The vendor mismanufactured the part initially, but the failure rates of the parts were very low (to the

It was an inspection failure from SpaceX's perspective. The vendor mismanufactured the part initially, but the failure rates of the parts were very low (to the order of several out of a batch of 10000 units), hence they weren't caught in batch testing. Also note that metal performance at cryogenic temperatures is often very different. Certain impurities that are largely harmless at room temperature can cause metals to be completely brittle at 90 kelvin. So testing every single strut would be very expensive, though SpaceX probably does that now regardless.