If the parent frame of the layout is the exact size it needs to be for things to fill it, it moves the end grid item to the next layer.
However, adding 1 pixel on to the parent’s size fits everything into the grid correctly - but there’s now a space between the edge of the frame and the grid item.
Happens both in studio and in-game.
To replicate:
Make a basic UIGridLayout setup, and set the parent frame to the exact size the contents should be.