UPDATE: I spoke today with the trailer manufacturer, I sent them my photos, and a link to this thread. They agreed that this is a manufacturing defect. They also volunteered that the same welder would likely have done all the hinge welds in a given trailer. They offered to repair it at no cost, and to inspect and re-weld (if necessary) all the other hinge welds on the trailer.
I will post back here after I have the work done. Thank you all for your good advice and experience.
2002 National Dolphin LX 6356
Workhorse W-22 chassis
Don't believe everything you think.