I've heard that the rubber engine isolators can cause something like this, and that would be my guess. I doubt there was any quality control issue from the factory.
Just re-do the welds that are there and VERY DEFINITLY do NOT weld around the tube, only parallel to it. If you look, you'll notice that none of the tubing is welded around, only longitudinally. Welding around it will weaken it severely!
It may be difficult welding between the engine and frame tube where, I'm sure, there was another bead - so it may be necessary to remove the engine to get the job done properly. But, hey, the splines probably need lubing anyway and pulling an engine isn't a big deal.