A reader at respected airline industry site Leeham News offered a comment that suggests they have access to Boeings internal quality control systems, and shares details of what they saw regarding the Boeing 737 MAX 9 flown by Alaska Airlines that had a door plug detach inflight, causing rapid decompression of the aircraft. The takeaway appears to be that outsourced plane components have so many problems when they show up at the production line that Boeings quality control staff cant keep up with them all.

Source: Boeing Whistleblower: Production Line Has “Enormous Volume Of Defects” Bolts On MAX 9 Weren’t Installed - View from the Wing

I’ve read that Boeing is no longer an engineering-led company.