I think Willie has the correct answer, those tubes needed relief to be placed and then completed.
As a note, someone mentioned laser/plasma cutting the fishmouths. I was into Karting deeply for some years, the manufacturers have a machine that feeds a length of tubing through the throat, uses a laser to cope the end rotates it and then bends the tube. Since the chassis parts are a single pc with multiples of bends and orientations. Fantastic to see the videos.
It is too bad that that joint will be painted or powdercoated, a work of art.
John
