Splitting closed polysurface creates strange shape

Rhino Rhino for Windows
1

Splitting the black closed polysurface with the red surface produces a strange shape at the bottom-looks like a bug to me, or please explain.
Using Version 6 SR8

cutaway%20block%20trimmed%20before%20split
cutaway%20block%20trimmed%20before%20split1437×695 61.2 KB

cutaway%20block%20trimmed%20after%20split
cutaway%20block%20trimmed%20after%20split1437×695 109 KB
cutaway block trimmed.3dm (150.5 KB)