Mc tool,
Yes they are both steel, but I thought a cause could be different alloys.
But in the meantime I've asked a professional welder and his first/immediate reaction was: "that is the work of a layman, most likely autogenous welded without adding any metal. Almost sure that the crack is just a shrinkage crack (due to lack of adding sufficient metal)".
He said that the fact that it is a longitudinal crack right in the middle of the weld together with a very thin and superficial weld gives it away.
"Just carefully grind the remaining weld away clean it and weld it properly and done".
According to him it is very likely that those cracks started to appear quite early on (he said the cracks would appear even if they were left on a shelf without additional stress.).
Well then we know that again.
Frank