When setting up a bitmap fill, you can rotate and scale it, and when replacing the image (especially if you modified the fill image and want to update it in designer) it remembers most of that, except the aspect ratio gets reset.
It seems this is done because it sets the aspect ratio to the aspect ratio of the image, but preferably it would instead set the aspect ratio as:
newFillAspectRatio = (previousFillAspectRatio / previousImageAspectRatio) * newImageAspectRatio
so that it remembers how you stretched the texture.