Hi Rich_Art, so it appears to be a bug somewhere in the triangulator.

If you export with "Output N-gons" the actual n-gon will get written despite the display being wrong. But if you export as "Output: Quads & Triangles" or "Output: Triangles only", then those areas will be missing in the output as well.

If you need to export triangles instead of n-gons, you can use the attached model version - here I've edited those areas to be made up of trimmed planes as Burr mentions above and those happen to avoid the triangulation bug.

I will continue to investigate the triangulation bug, it's been quite a while since I've seen one of those.

But the attached model should get you around this issue for now anyway.

- Michael