Revit, SectionView (viewing from above) created from FloorPlan is mirrored

Question

I try to create a SectionView out of FloorPlan. The viewing direction is from above, actually the same as in the FloorPlan. The SectionView created is mirrored:

resulting section view

Here is my code:

        //uiapp is UIApplication, ass is input assembley, danglerad is input angle radian
        FamilyInstance fibox = GetPlaceholderBoxFromAssembly(uiapp, ass);
        double dw = bbox.Max.Subtract(bbox.Min).X;
        double dd = bbox.Max.Subtract(bbox.Min).Y;
        double dh = bbox.Max.Subtract(bbox.Min).Z;

            snameprefix = "up";

            max_pt = new XYZ(dw, dd, dh); 
            min_pt = new XYZ(-dw, -dd, -dh);

            bbox.Max = max_pt;
            bbox.Min = min_pt;

            trans = Autodesk.Revit.DB.Transform.CreateRotation(XYZ.BasisZ, danglerad);
          trans.Origin = mid_pt;

        try
        {
            bbox.Transform = trans; 
        }
        catch { }           
            
            
        using (Transaction tr = new Transaction(uiapp.ActiveUIDocument.Document, "CreateSecPlan"))
        {
            tr.Start();
                viewsec = ViewSection.CreateSection(uiapp.ActiveUIDocument.Document, viewfamtypeid, bbox); 
                viewsec.Name += $"_{ass.Name}_{snameprefix}";
                viewsec.DisplayStyle = DisplayStyle.HLR;
                viewsec.DetailLevel = ViewDetailLevel.Fine;
                viewsec.ViewTemplateId = templid;
                viewsec.Scale = iscale;
                
                viewsec.get_Parameter(BuiltInParameter.SECTION_COARSER_SCALE_PULLDOWN_METRIC).Set(1);

                tr.Commit();                    

        }
        
        using (Transaction t3 = new Transaction(uiapp.ActiveUIDocument.Document, "MirrorView"))
        {
            t3.Start();

            Plane pln = Plane.CreateByNormalAndOrigin(viewsec.ViewDirection, viewsec.Origin);
            ElementTransformUtils.MirrorElements(uiapp.ActiveUIDocument.Document, new List<ElementId>     { viewsec.Id }, pln, false);
            t3.Commit();
        }

Can anyone tell me, what's wrong here?

I've also tried: viewsec = ViewSection.CreateDetail(uiapp.ActiveUIDocument.Document, viewfamtypeid, bbox); and view's CropBoxElement mirroring. Both to no avail.