I have an OBJ file. When I view that in Microsoft 3D Viewer, it looks ok. Same OBJ file, when I view in Babylon Sandbox, it looks different. At the curve part of the meshes are show triangles instead of smooth surface.
I have attached the image for the reference.
In Babylon Viewer:
It sounds like the triangles are reversed for some reasons in your obj. Sharing the model or a repro in the pg could help, also there are a few toggle on the obj loader helping to smooth normals and so on which need to be intentionally turned on at the dev discretion depending on the case.
The winding of some faces seems inverted, if setting clockwise instead of counterclockwise on the materials some missing faces appear (and others disappear)…