When Generating fill, while a selection is active, Nano Banana Pro messes with the aspect ratio.

Nano Banana Pro scales results without keeping aspect ratio. It seems to take the aspect ratio of the canvas and scales that to the bounding box of the selection. The result of a Generation in a selection is, that the generated fill does not match the original size and aspect.

In the screenshot you can see what was selected (the whitish silhouette), and how the result is squeezed.