Bug? Scaling sphere at one direction.

Hello everybody.

The following simple program does not work properly.
I think it is bug.

void OnDebug_test6(Handle_AIS_InteractiveContext myAISContext)
{
// Create Sphere
gp_Pnt aCenter(0,0,0);
TopoDS_Shape tShape = BRepPrimAPI_MakeSphere( aCenter, 50.0 );

// Scale down at Z axis only
{
gp_GTrsf aTrsf;
gp_Mat rot( 1.0, 0, 0, 0, 1.0, 0, 0, 0, 0.5 ); // Only Z scale down 50%
aTrsf.SetVectorialPart(rot);
BRepBuilderAPI_GTransform aBrepTrsf(tShape,aTrsf);
tShape = aBrepTrsf.Shape();
}
// Display
Handle(AIS_Shape) aShape = new AIS_Shape(tShape);
myAISContext->SetDisplayMode(aShape,1);
myAISContext->Display(aShape, Standard_False);
}

Almost everything goes disappear and only wireframe is hilighted when mouse moves to the sphere.

http://www.fisco21.com/img/OCC/20090701.jpg

Some of my trials.

1. I try converting sphere into nurbs by BRepBuilderAPI_NurbsConvert, but the result was same.

2. I build the shape by BRepPrimAPI_MakeRevol with 360 degree,
arbitrary cross-sectional contour such as half of circle, half of ellipse, spline curve,
the result was also same.
but the donuts like shape was correctly scaled.

3. I try shape healing function ShapeFix_Shape, but the result was same.

Any suggestion and opinion would be appreciated.