mesh boolean union (MUnion )

Hello all,

I have recently using Munion to cut the intersection of 2 mesh surfaces. It is strange for me that grasshopper sometimes delete the intersection part from 2 parts. However, it sometimes subtracts the mesh part 2 from mesh part 1, and nothing from mesh part 2 remains in my geometry. Could anyone help me how this happens, and how I can prevent this contradiction?

Thanks,

Maryam