mesh partial render behind clear glass

This problem has occurred in a few of my building renderings and I have still not figured out why after many tries.  Microstation version is V8I SS2.

A site grade outside the building window is a mesh element with a grass material assigned to the layer-color.  The window frame system is modeled with rectangular solids having a material assigned to the frame layer-color.  The glass in the frame is modeled as a solid with a uniform thickness of 1/2".  A clear glass refractive material is assigned to the glass layer-color.  I have included screen pics of the glass and turf definitions:

The problem is that the mesh outside the building partially disappears in the rendered result when the glass level is on. The following image shows the problem which occurs roughly right of the room corner.  On the left the grade mesh appears correctly.  On the right the mesh disappears and trees come from the attached background .jpg:

The following is the render result with the glass level switched off:

Just a clarification, the bright white surface is a retaining wall in the mid-view.

I have included a plan view just for reference:

Has anyone seen this problem and found a solution for it?

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