Anyone ever had this kind of issue with the header ?
That's a first I've seen! Let me make sure the factory / MattP sees this and, we can go from there.
I see. Did you cut the pipe? There's one spot that looks like the metal was torn, probably the location where one of the initial tack welds was done, but the rest looks like a cut, and it traditions from one side of the weld to the other. It's just a strange looking failure. Usually when a TIG weld fails from lack of penetration you'll still have some melt, otherwise it would just fall apart as soon as you lifted the part off the table or out of the jig.
I'm just trying to figure out what the pink lines on it were for. I'm guessing for alignment reference. Just trying to determine what all happened, and when, so I can best understand the nature of the failure.