The more I read about this whole issue of frame failures the more convinced I am the fault lay in the original hinge design.
If you look at the latest latches both OCL and FBL 2.0 you can see how they clamp together more securely than earlier types minimising torsional stress on the frame through movement.
The FBL latch is particularly good I think as one hinge surface sits inside the other rather like a door when it's shut into it's frame.
I'm less inclined to believe it was a welding fault as the welding appears to be exactly the same on newer models, all that's changed is the frame latch.
I've see some of the paperwork that's given by some manufactures to stockist who sell folding bikes, and there's great emphasis on the need to get across to the customer the importance of making sure those hinge bolts are secure. To quote one cycle manufacturer "
Advise that an incorrectly adjusted Folding Joint may cause Frame damage or result in user injury " that's clear enough to me, actually Tern, Dahon and others all issue similar advice. In fact there's more emphasis on the latch being correctly adjusted than on making sure your wheels are bolted on securely or bars are tight !