You are using a DirectShape element that is categoried as a Mass. So Revit does not see that as a Mass family instance. DirectShapes are great to bring complex geometry into Revit but they are only categorized under the given Revit category and do not adopt the behaviour. See this guide
Use the New Family component to create a Mass family and place the Brep inside the family. Then place an instance of this Mass family where you want:
Run the Grasshopper definition in an empty model and make sure the Masses are visible:
Family as Mass.gh (8.3 KB)
Now Revit knows this is a Mass family and can pick up the surfaces:
