This may be intentional, but when taking a 16x9 render and expanding it to a square, your system adds to the original image dimensions and does not scale down the resolution of the original image. However, when I take my original and compare it to the expanded version, there are what look like jpeg artifacts in the "unchanged" portion of the expanded image. I understand the expanded portion not being perfect, but the entire image is worse off for running it through the gen fill process.
There seem to be no issues like this in Photoshop beta.
I love the tool, and it's getting better. Thank you for taking the time to read this.