I have noticed corruption at other times as well. I started editing an image, then clicked another image before clicking “done”, realized I had done that and clicked back to the original image. I clicked “done” and then went to the second image but now the second image loads almost entirely black. In this case, the thumbnail is fine and the image is the corrupted element. I am thinking this is some sort of problem with Apple Raw processing … maybe some kind of race condition and the “working copy” of the image or thumbnail gets messed up.