There seems a bug in exporting .obj format in Rhino 6.
For meshes generated from curved surface, everything works fine.
For meshes generated from a planar surface, the exported .obj file only contains one “f …” line.
Rhino 5 work fine for that.
This creates issues for existing importers of obj in other geometry processing library.
(My report error when importing the one from Rhino 6.)