I have recently got a Mamiya M645 1000S and did a test with one of the rolls to see if the camera was working. At the top the images is a light distortion. Would anyone know what the problem could be? I've attached a couple of images.
That seems too sharp of a division to be a light leak. I'd say not enough chemicals in processing. I'd shoot another roll of say B&W and process again.
I had this issue with a M645 (essentially the same camera without the 1/1000s shutter speed). This is a light leak related to the back of the camera. Check the back to make sure it closes properly. In my case, the back was bent and I repaired this issue by removing the back and replacing it with the back of a camera with failed electronics. The back is very easy to remove.
Had a 1000s with similar problem (I'm assuming this is running across the film, not lengthwise if the frame dimensions shown are representative). Not as severe but it turned out to be the back. New foam sorted it out.