I tried SplitBrep in Rhino 5 and it works fine on Surface and Polysurface but Rhino 6 cuts only polysurface. Both object is cylinder but Cap was applied on tube surface to close the geometry , the other one is pure polysurface by joined surfaces.
SplitBrep test.3dm (144.4 KB)
This post is part of the SplitBrep discussion, TrimBrep does not work - #4 by onrender