Why can I see through compound cells

If I place a compound cell (window) so that it perforates a compound wall; and the cell is so constructed that it should mask any of the wall around it when seen in section, why am I seeing the forward view lines merging with the cell during a DEM extraction?

The cut plane is calculating correctly, but  the forward view of the compound cell is not being calculated, letting the walls forward view show through.

This behavior does not occur in a dynamic view cut, but then the cells parts&families do no unify with the wall where they should.

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