SketchUp has difficulty forming very small faces (under 1mm or 1/16 inch). When you round a corner with a high segment count, the resulting faces may be too small for SketchUp to "heal," leaving a gap or crack.
If the top face of your object isn't perfectly flat, standard rounding tools will fail to calculate how the curve should meet the surface, leading to geometric "tears". sketchup round corner crack top
Use the Preview mode in FredoCorner to see if your offset is too large. If the preview shows red or overlapping lines, reduce the radius until they disappear. SketchUp has difficulty forming very small faces (under