That much of a crack and on the bottom of the top tube ? Perhaps most/all tubing is like this ? Perhaps the tubing when welded together was oriented like that intentionally because it makes no difference that way ? Effectively, a bow on the sides might indicate a frame turning a direction either way, but top & bottom, it's straight enough. Even on the sides, is it within tolerances of steel tube manufacturing specs ? I'd measure other bikes and if it's found to be the case across the board, it can't be a real problem ?