In my experience on tanks and fighting vehicles ballistic welders are highly trained and certified. Each & every one, even DEI hires, knows a good weld from a bad one, ditto for the inspectors. I suspect the Navy welders are the same. One or two bad welds could be the result of having a bad day or a problem with faulty equipment...but not at the same facility on 20-something ships/subs over several months.
The only thing I can think of to cause that many faulty welds would be the use of the wrong rods or filler material.That wouldn’t show up on xrays or even the inspector looking at it. That wouldn’t be the welders fault or it is possible the filler material was substandard, not to specs and possibly mislabeled by manufacturer, maybe even on purpose so they say knowingly but they do not mention who knew.