I assume that it slides over the standard wedge so not likely enough room to weld the inside of the rectangle.
I think you may be correct, what I think caused the failure is the pressure applied by the wings during splitting. It's an easy fix just grind and re weld, the problem is the other side is going to fail also from what I see in the Picture.
I would take a grinder and cut the welds on the other side almost into so I could get to the core. The first pass should be right down the center bonding the two edges of the base metal, next passes should be on the two edges of base metal, then you could weld the center with a pass bonding the 2 edge passes. repeat till you form a corner. This way you will bond with good penetration the base metal and the weld that forms the corner. The problem is they made a corner weld but didn't get good penetration on the base metal on each edge. That's what is called an full open corner and it's a strong weld for this application if done right. Going back and looking at the picture it looks like the weld failed itself and they didn't weld the corner weld all the way in, it was just welded 1/2 the way and that is why the weld failed. Apply a U Groove corner weld to the welds and I bet it holds forever, a weld should never fail in itself, it should be even stronger than the base metal.
Mild Steel has a tensile strength of 58,000–80,000 psi, depending on the amount of carbon incorporated. When using a Mig I use ER70S-6 and 95/5 argon/oxygen mix on structural steel, which produces a tensile strength much stronger than the tensile strength of mild steel. It this case the mild steel will fail before the weld.