I suspect it is porosity as well as cold lapse. X-ray would have picked this up upon inspection unless the inspectors were incompetent or paid to look the other way.
A nuke plant here in Texas had substituted inspection x-rays inserted into files and they got caught after the fact with post build inspections prior to start up. Cost rate payers ALOT of money to fix.
Post #12 says it best, IMO. Quality control is paramount.