I think the flaw is in the idea of a tube welded to a flat hinge face with very little structural shape to the hinge face that would hold the tube more securely while allowing the weld to be rather more than a thin line of metal put down. The hinge is in the middle of the bike right between the wheels, and that is the point of maximum stress. The rider's weight on the saddle in front of the rear wheel, exerts a strong downward force on the tube. Add some extra dynamic stress of riding on uneven road, and fatigue sets in quickly. A tiny flaw at the bottom such as a corner would be a stress riser where a crack would begin. With a very strong weld design which has lots of extra strength over and above the necessary amount to carry the rider, the crack would not form.

For sure a safer design is for the hinge to be a 3D piece that gets inserted into the tube after which a weld is put down. This tactic would allow the weld to be convoluted for extra strength. And it doesn't need to be like that everywhere, just the bottom where the maximum stress is. Heck even an extra reinforcing plate welded over the first weld at the bottom would be a massive improvement.