When I select the circle option in an image stack, it makes an oval instead of a circle.
In order to get a perfect circle I have found that your image needs to be a square, not a rectangle. Try cropping the image in a square and see if that helps.
Exactly as @Steve_J mentioned. The image needs to be square shaped. The stack cannot do the cropping of the image to make this happen so you’ll have to do so before dropping it into the stack.
Thanks. I will give that a try tomorrow.