You show that the 4-image merge works and the 6-image merge doesn't. I wonder if the problem isn't the number of images, but rather the content. The difference that jumps out to me is that the 4-image merge doesn't include the window. That feature is overexposed relative to the rest of the scene, and that may affect the blending if PS is trying to compensate for different exposures. What happens if you try to merge the 4 images on the right, including the window image?