There are many possibilities… the camera eyepiece diopter may not match your eye, the viewfinder screen may not match the actual focal plane, the camera lens mount flange may not be perpendicular, a lens element may be out of alignment…

The only way to narrow the search for the problem is to do thorough, documented testing.

I have the 50mm f1.2 AIS, and I doubt there is a problem with the actual lens. More likely, a slightly out or adjustment viewfinder screen or eyepiece diopter.