If the top surface of your model is even slightly warped, twisted, or un-planar, the rounding algorithm cannot cleanly calculate where the curved edge meets the top face. This results in stray geometry and open meshes.

Because SketchUp struggles with tiny faces, scaling your model up before applying the tool is the most reliable solution. Select your object. Make it a or Component .

SketchUp cannot create microscopic faces under 1/16 of an inch (about 1mm). If your rounding radius is small, the tool fails to generate the tiny new geometry, causing the top face to rip open.

Here is the "story" of why this happens and how you can fix it. 1. The Mystery of the "Tiny Face" Limit