It would appear that you’re using stretched anchors for each button with some additional offset data, and so the way the math works you can actually cause the buttons to get a negative layout size if you’ve embedded position offsets as well as stretching anchor data into your buttons. eg. If you button is always between .75 and 1 of the screen, and has an offset of 100px from those anchor points, then if the size of the screen is small enough, those offsets can cause the buttons to invert themselves - thus the weird borders.
The cure is to just use anchor points, make sure you don’t embed offsets from those anchor points if you don’t want constant pixel offsets.
The best cure is to use a mix of relative and fixed layout. Instead of trying to anchor each button, drop down 1 Horizontal Box, anchor it to the corner, you don’t even need to stretch anchor it. Then just place your buttons in the horizontal box.