The BorderMode property of ViewportFrames always appears to be outset, even if it is set to middle or inset. This bug only seems to happen with viewportframes exclusively. I first noticed this bug several days ago. To reproduce, simply place a ViewportFrame within a screengui or any other gui object and set the BorderMode to middle or inset. Also recommended to increase border thickness so the issue is more obvious.
This is correct:
This is not: