This was really an inspection error. A human can miss a weld, too, so manufacturing lines rely on inspections. Someone was signing off on work that wasn't done. Whether a human or a robot was supposed to do that work is a nit.
The affected vehicles were built between 13th and 21st.
Not sure how long QA takes for a vehicle, but they found it within a week.
Or a camera that takes a picture of each vehicles undercarriage. They probably thought that was unnecessary until now.