Material Unknown

Why is Revit uploading an UNKNOWN Material and creates this possibly on download?

We cannot read all materials from Revit the same. Often materials of wall or floor layers are set to <By Category> but the category doesn't have any material applied. This is the moment when we are defaulting to Unknown. Also if this is the setup on download, we cannot apply any materials. This is why you can ignore the Unknown material on download into Revit.