To best describe the bug, it is best to show a couple pictures of the bug in action.
In the following picture, you can see white outlines appearing on top of the light wooden part when the grassy part is being resized. However, the grass is below the wooden part.
What is even more curious is that the two parts are not even touching as can be seen from this side view.
I looked into the behavior a bit more and found out what is causing this bug. As it turns out, the cause of the problem is the combination of the thickness of the grass and the position of its left edge. Apparently, the left edge of the grass part is positioned exactly one stud above the wooden part, which is also equal to the thickness of the grassy part. In the following picture, I placed a black part with a thickness of 1 stud on top of the wooden part. As you can see, the left edge of the grassy part perfectly touches the top surface of the black part.
When the left side of the grassy part is resized, the white outlines disappear because the height of the edge has changed, as can be seen here.
Here is a simple reproduction file:
Outline_Repro.rbxl (12.6 KB)
Simply resize the grassy part along the X-axis (grab the red handles) and you will see the white outlines appear. Whether or not the white outlines appear also depends on the ‘Join’ mode. If it is set to ‘never’ the outlines do not appear, so change this button to ‘Surface only’ instead:
