This bug will be fixed (read: already is fixed) in SR1 for Rhino 6. Once Rhino 6 is out SR1 will follow pretty soon. The bug is the same as the reflection bug you reported earlier ( Reflection of groundplane? Monochrome )
Since the change requires huge code-base changes (merge of upstream Cycles into RhinoCycles) this close to release it will go to the first SR. Otherwise we will never be able to stabilize the release… 
