I’m using Defold Orthographic with a custom viewport. My window size is 960 x 540 (16 : 9), but the viewport is 520 x 520 (1 : 1). The resulting game world is stretched (or squished) on the sides. The colored rectangle is 256 x 256 pixels without the viewport change.
That’s probably a bug. The viewport functionality is not very battle tested, and does apparently not work when the aspect ratio of the window and viewport aren’t the same.