[MechEng83]

This is actually a common mistake when using planes in any 3D CAD package.
Even though the visual representation of the working plane appears to be limited to a specific rectangle, in actuallity, it goes to infinity in every direction. Thus, when you try to use the plane to "split" one end, it is splitting along the entire plane.

I suggest either creating an actual surface in the vicinity of your split or a 3D extrusion, depending on what your final intent is.