Just to clarify on my previous post: With a flat face welded to a perpendicular tube, there is only a normal force on the weld.
Add a section where the weld tracks around a finger-shaped protrusion of a 3D hinge, to break the weld now also involves a shear stress, far stronger than a weld stressed with only a normal force.
true but so is the downtube at the headtube .... more or less anyhow.... the first framebuilder just screwed up