Weld is on in all the examples. The think the reason things are going wrong is holes are made by non coplanar polygons. In this case there are 5 vertices on the fifth one is the pointed edge that is bridging the vertical fillet with the side of the knob.

Michael, I've searched through all the settings of Modo and can't find one that is going to fix this geometry problem. Can you please let me know, when tri + quads are selected as the export type do you generate any non co-planar quads? I suspect not, and this is why that export renders fine, but the n-gon doesn't.