Transparency bug when using multiple renderers and dual depth peeling.

I recently updated my project from VTK 7.0 to VTK 7.1 and found a couple of issues related to transparency (see #17042 (closed)).

I'm rendering a mesh in front of an image. To do so I have two renderers in two different layers, one in the background for the image and one for the mesh.

The following code works with the legacy vtk depth peeling but not with dual depth peeling :

#include <vtkVersion.h>
#include <vtkActor.h>
#include <vtkCamera.h>
#include <vtkImageCanvasSource2D.h>
#include <vtkImageActor.h>
#include <vtkImageData.h>
#include <vtkJPEGReader.h>
#include <vtkPolyData.h>
#include <vtkPolyDataMapper.h>
#include <vtkRenderer.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkSmartPointer.h>
#include <vtkSuperquadricSource.h>
#include <vtkPlaneSource.h>
#include <vtkMatrix4x4.h>

#include <vtkProperty.h>
 
int main(int argc, char *argv[])
{
  vtkSmartPointer<vtkImageData> imageData;

  vtkSmartPointer<vtkImageCanvasSource2D> canvasSource =
  	vtkSmartPointer<vtkImageCanvasSource2D>::New();
  canvasSource->SetExtent(0, 100, 0, 100, 0, 0);
  canvasSource->SetScalarTypeToUnsignedChar();
  canvasSource->SetNumberOfScalarComponents(3);
  canvasSource->SetDrawColor(127, 127, 100);
  canvasSource->FillBox(0, 100, 0, 100);
  canvasSource->SetDrawColor(100, 255, 255);
  canvasSource->FillTriangle(10, 10, 25, 10, 25, 25);
  canvasSource->SetDrawColor(255, 100, 255);
  canvasSource->FillTube(75, 75, 0, 75, 5.0);
  canvasSource->Update();
  imageData = canvasSource->GetOutput();

  // Create an image actor to display the image
  vtkSmartPointer<vtkActor> imageActor =
    vtkSmartPointer<vtkActor>::New();

  auto texture = vtkTexture::New();

  vtkPolyDataMapper* mapper = vtkPolyDataMapper::New();
  vtkPlaneSource* plan = vtkPlaneSource::New();
  mapper->SetInputConnection(plan->GetOutputPort());
  imageActor->SetMapper(mapper);

  imageActor->PokeMatrix(vtkMatrix4x4::New());

  texture->SetInputData(imageData);

  vtkProperty* property = imageActor->GetProperty();
  property->RemoveTexture("texture");
  property->SetTexture("texture", texture);

  imageActor->SetScale(100., 100., 1.);
  imageActor->SetPosition(50., 50., 0);

  // Create a renderer to display the image in the background
  vtkSmartPointer<vtkRenderer> backgroundRenderer = 
    vtkSmartPointer<vtkRenderer>::New();

  // Create a superquadric
  vtkSmartPointer<vtkSuperquadricSource> superquadricSource = 
    vtkSmartPointer<vtkSuperquadricSource>::New();
  superquadricSource->SetPhiRoundness(1.1);
  superquadricSource->SetThetaRoundness(.2);

  // Create a mapper and actor
  vtkSmartPointer<vtkPolyDataMapper> superquadricMapper = 
    vtkSmartPointer<vtkPolyDataMapper>::New();
  superquadricMapper->SetInputConnection(superquadricSource->GetOutputPort());
 
  vtkSmartPointer<vtkActor> superquadricActor = 
    vtkSmartPointer<vtkActor>::New();
  superquadricActor->SetMapper(superquadricMapper);
  superquadricActor->GetProperty()->SetOpacity(0.4);
 
  vtkSmartPointer<vtkRenderer> sceneRenderer = 
    vtkSmartPointer<vtkRenderer>::New();

  sceneRenderer->SetUseDepthPeeling(1);
  sceneRenderer->SetMaximumNumberOfPeels(8);
  sceneRenderer->SetOcclusionRatio(0.);
 
  vtkSmartPointer<vtkRenderWindow> renderWindow = 
    vtkSmartPointer<vtkRenderWindow>::New();
 
  // Set up the render window and renderers such that there is
  // a background layer and a foreground layer
  backgroundRenderer->SetUseDepthPeeling(1);
  backgroundRenderer->SetMaximumNumberOfPeels(8);
  backgroundRenderer->SetOcclusionRatio(0.);

  backgroundRenderer->SetLayer(0);
  backgroundRenderer->InteractiveOff();
  sceneRenderer->SetLayer(1);
  renderWindow->SetNumberOfLayers(2);
  renderWindow->AddRenderer(backgroundRenderer);
  renderWindow->AddRenderer(sceneRenderer);
 
  vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor = 
    vtkSmartPointer<vtkRenderWindowInteractor>::New();
  renderWindowInteractor->SetRenderWindow(renderWindow);
 
  // Add actors to the renderers
  sceneRenderer->AddActor(superquadricActor);
  backgroundRenderer->AddActor(imageActor);
 
  // Render once to figure out where the background camera will be
  renderWindow->Render();

  // Set up the background camera to fill the renderer with the image
  double origin[3];
  double spacing[3];
  int extent[6];
  imageData->GetOrigin( origin );
  imageData->GetSpacing( spacing );
  imageData->GetExtent( extent );

  vtkCamera* camera = backgroundRenderer->GetActiveCamera();
  camera->ParallelProjectionOn();

  double xc = origin[0] + 0.5*(extent[0] + extent[1])*spacing[0];
  double yc = origin[1] + 0.5*(extent[2] + extent[3])*spacing[1];
  //double xd = (extent[1] - extent[0] + 1)*spacing[0];
  double yd = (extent[3] - extent[2] + 1)*spacing[1];
  double d = camera->GetDistance();
  camera->SetParallelScale(0.5*yd);
  camera->SetFocalPoint(xc,yc,0.0);
  camera->SetPosition(xc,yc,d);

  // Render again to set the correct view
  renderWindow->Render();

  // Interact with the window
  renderWindowInteractor->Start();
 
  return EXIT_SUCCESS;
}