This is far from being a crack, I would say it would be more in line with terrible design. I guess today's engineers never heard about the Drilling Rig Collapse in the north sea years ago. Round tubing does not make strong supports, the Egyptians figured that out over 3000 years ago. Triangulated structure would have been a much better source. Luckily people on the ground got some very good photos to shut this death trap down before it killed a lot of people. That train is really moving with some incredible force and the shift on the structure was just killer, no pun intended of course.
The north sea rig failure was exactly the same as this roller coaster failure. A large round section of one of the uprights had been modified with a hole being cut and welding a flange modification in place, it weakened the one upright and the rest is history. If you look at the weld on the coaster with the angled attachment you will see what happens to tubing when a lot of welding is near a stress point. The welds were great but not the original strength of the steel tubing, that was destroyed during the welding process.
I've read about this elsewhere. As you said, this was not discovered during the required periodic safety inspections...customers spotted it. And it had been reported by customers a couple of years ago. Scary stuff. And it belies the story that the stationery rides are safe.