Can you send a screenshot of the DWG settings from Navisworks (N>Options>File Readers>DWG (and the Convert Object Properties subsection)?

Also if you are opening Navisworks and appending the DWG from fabrication, try exporting from FabCADmep to a different format, DWF or similar then append that into your model. One option is you could export to MAJ, import MAJ into Revit, then link the Revit model. Unfortunately Revit doesn't read MAJ (wish it did).