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:
Hi @Protectorates - thanks for reporting this issue and for providing screenshots demonstrating the issue! I’ve created a ticket for us to investigate and resolve this.
I am in the process of checking over bug reports and following up on some bugs that haven’t received any activity in a while.
Is this issue still occurring or can you confirm that this bug has been resolved?
This issue is still occurring for me.
Alright, when there is an update to this issue i will let you know
Bump! The issue is still happening as of today.
Was hoping to use BorderMode.Inset on my viewport frame, guess not 
this is still happening, somehow