as the bright part is always in the same place - and has a very sharp line, I do not think it is a light leak. Could it be that the shutter gets stuck? I do not know the mamiya 645 but i have seen a similar problem on a 35mm SLR. The shutter did not run smoothly, and therefore the negatives where overexposed in part of the image.
maybe the experts around here will have a better idea of the reason for this problem.
That's my guess too. The line is too sharply defined to be a light leak. I have an old Canon FTb that exhibits the same problem, though in a different orientation since the shutter runs horizontally across the film gate rather than vertically as in your camera