9 - Slice not working as expected?

I’m trying to create a 9-Slice and the stretching on the X and Y axis isn’t doing what I expected.
It’s stretching outwards, but isn’t stretching inwards.

For example here in my image for testing sake:
800 x 400

I set the SliceCenter to be {20, 20, 780, 380}, If I make the image smaller in Roblox I get this result:
image

Here is how this looks using Unity

Is this intended behaviour or am I doing something wrong?

I know @SolarCrane helped develop 9-slice but I can’t find this behaviour documented anywhere.