This is almost certainly a process problem vs a design problem. There is a tool that engineers use, called a failure mode and effect analysis (FMEA). In that tool, you try to list all possible failures and rank them based on their severity, probability of occurrence, and ease of prevention. On the design side, it's easy to minimize the well failure, failure mode. You design in location trabs, or other feature to ensure the party is properly positioned. You does the correct materials to ensure the welds will stick. That stuff is old hat.
Process wise, you are dealing with an entirely new assembly process. New fixtures, new parts, possibly new robots, new everything. Assembly is incredibly complex in today's automobiles. Lots of error detection and prevention going on, not to mention the design continue to get more and more complex. There are entire teams dedicated to process design. On the process side they missed something that is allowing those welds to not be correct. Its probably already corrected too. Those guys are smart, and I'm sure they figured out how to fix it.
Crappy thing for FCA is that it was probably something really simple that was just overlooked.