Looks like a doggy weld to me … ie the crack comes from the weld which may have been badly done resulting in a microscopic crack where the weld joins the tube - such cracks can make a tube MUCH weaker. Maybe it was a rooky welder ? , who’s work was not 100% checked - maybe robot welding would be better ? (but there again many bike companies promote ‘hand welded’ as an advantage).
At least Strida Stand by their frames, with good guarantees and replace any faulty ones - good result.