I see. So either terminate the tubes separately, then add a gusset or reinforcement welded section.

Is the problem the fact that the hinge is the connecting part of the frame? Ie the hinge plate IS the structural section joining the parts.
Rather than the two tubes meeting each with a hinge plate around it to aid conherencey. Downtube ,and brompton seem to do the later. More likely to come undone but not to stress crack at bottom?

None of my bikes have front folding sections anymore.