during import of .obj (.mtl) when nanite is turned on, very fine thin line meshes (ie circles thin lines along sphere around main object) will have strange reduction of polygons, even if the polygon reduction is not turned on. when nanite is turned off, all shapes appear like in 3d model editor and in exported file.
is nanite being culling triangles harshly. note: keep triangles and others are at default values 100%.