No transparency between QCustom3DVolume of the same Scatter3D

I would like to display on a single Scatter3D a couple of objects instantiated from a derived  QCustom3DVolume class (here 'BallVolume').  In this example, these objects are balls of different sizes, colours and transparencies, but they can also have a random shape.

You can see in the images below that the objects remain opaque to each other whatever the level of transparency I apply.

I apply transparency in the definition of the color table and also by playing with the alphaMultiplier parameter. I've also tried the preserveOpacity parameter.

Is this the normal behaviour, a limitation or a bug from  Scatter3D ?