Funky result when adding thickness to surface

In this file, I have a poly surface:
Problem Thickness.3dm (3.3 MB)

If I have no thickness for the surface, it looks like this:

If I add thickness, I get this:

Looks like a mesh issue / file tolerance. Use custom mesh settings and increase the file tolerance. The default Rhino mesh settings are worthless. Another possible cause is the model being far from the origin.

I just clicked CUSTOM on the mesh settings and the mess went away.

You’re welcome.