Are you sure you are saving your .osm project the right way? This is done by clicking the icon in the OS toolbar and NOT using File>>Save/Save As in the SketchUp Application. Various toolbars for OS can be made visible using the View>>Toolbars option... Saving the file in the SketchUp application does not save any of your Openstudio features, which I think could be one reason why your envelope is 'disappearing'.