I will try this. But that doesn't explain why the problem only happens on the long diagonals.

If I change the object by adding a hole in the middle of the flat piece to cause the diagonals to be shortened by half, then there is no issue on the shortened diagonals. The full length diagonals on either side of the hole bow up as usual. So that suggests it is not the fan, nor the geographic location on the bed, but rather just the length of a single extrusion thread.

Jim