The designer had previously drawn a center guide line across the top face (to align leg holes). That guide line was a hidden, un-deleted edge. The Round Corner tried to round across that internal edge, creating a crack.
, which is a more advanced version of the tool designed to handle complex corners and non-planar faces more robustly. SketchUp Community sketchup round corner crack top
Round corner doesnt work correctly - Extensions - SketchUp Community The designer had previously drawn a center guide
Those unsightly seams, overlapping triangles, and "pinched" vertices on the flat top surface can ruin a otherwise perfect rendering. But don't worry—this isn't a bug in Round Corner; it’s usually a geometry logic issue. creating a crack.