This is the expected behavior.
In ARCHICAD not all element types can react to their relative position to the Floor Plan Cut Plane. The Mesh is such an element, you will always see a symbolic representation of it in the Floor Plan Viewpoint and it will not be displayed differently even if the Floor Plan Cut Plane cuts into it.
GRAPHISOFT is slowly working toward making elements smarter in this regard, but it is not there yet fully.
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