This is most likely an alignment issue rather than a defect in the fisheye effect itself. If the lens’ optical axis isn’t perfectly perpendicular to the sensor, the circular image can appear shifted toward one side of the frame.

With an adapted setup, the adapter is the first thing to suspect: it may not be seating perfectly flat against the camera mount, the lens mount, or both. A tilt or flange misalignment can move the image circle off-center.

So between the two, the adapter or mount alignment is the more likely cause than the lens.