I think that my situation may not quite match the conditions you describe:
1) I *think* this worked in the past, but am not sure.
2) I was shooting a mountain ridge square in the middle of the picture. I have focusing set to a single area in the middle of the picture, so the square wuld have contained a dark mountain bordering a bright scky - which should cause very high contrast.
3) With focusing set to a single dot in the middle, I wouldn't expect holding the camera horizontally or vertically to make any difference.