Per API 7-2, the shoulder contact face is required to be plane and square with the thread axis within 0.002″.
You can detect a severely out-of square shoulder by sweeping a thread profile template up the thread’s helix until it butts up against the face of the shoulder. A visible or measured gap in excess of 0.002″ is an indication that the shoulder squareness may be out of specification.
If this occurs, first double check the taper for verification, and then check the alignment of your cutting tool.