“Weak spots in metal” means manufacturing defects. Aircraft drivetrains are not made from metal parts found on the side of the road outside of KSP, extensive treatments and testing goes into all of them.
manufacturer Bell Flight to find fixes for the critical system, which has had some components wear down earlier than the military expected.
I would guess the military expected it to last as long as it should because that was in the spec sheet. This sounds like it is all on Bell.