[CONNECT C++] DGN mesh vertex normal is lost

Dear experts,

I create a DGN mesh model.  However, I can't get the mesh vertex normal by API PolyfaceHeader::Normal(). In result, the converted mesh looks not smooth. My question: Should all mesh data in the MicroStation carry the vertex normal data? Is there any API to generate the vertex normal for the mesh body?

In MicroStation,

My result(not smooth),