I found this footnote on the page relating to the contour extraction method.
It appears that this method is not suitable to straight shapes like waveguides.
I have had similar problems extracting a fully etched grating coupler geometry where I am still seeking help as I posted in another forum entry here
It seems that exporting to GDS either based on the contour method or based on geometric object height are both insufficient for the instance of a filly etched grating coupler based on the Lumerical example files for the 3D inverse grating coupler design.
Can you think of any other way of extracting the proper geometry to a GDS file with no error?
Thanks again