This is very interesting. I have been using 2.81a for a while now and imported many OBJ's from Zbrush and 3dsmax but have not seen anything like this. I'm curious if this is actually an import error or a display driver issue? Can you verify that the geometry faces are completely absent from the mesh entirely, or can you select the faces that are supposed to be there but they're just invisible? You mentioned that the lines and vertices are still there so I'm assuming the faces must also be there, just that you can't see them? If you are in face selection mode, can you select any of those missing faces?

I did a quick search for this problem but I haven't found anything useful. If you are able to pose a screenshot with wireframe edges visible it might help. Or better yet post the .blend file with a mesh afflicted with the issue so we can have a look first hand.

I'm assuming also that you already inspected the surface normals and the geometry is actually gone?

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