Understanding costmap_2d tutorial: Updating master_grid

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I'm trying to understand this tutorial about the new layered costmap structure in Hydro. I successfully integrated the grid_layer and it runs out of the box just fine.

I then started to work with a recorded bag-file and the used layer structure in the yaml file looks like

  plugins:  
    - {name: static_layer, type: "costmap_2d::StaticLayer"}
    - {name: obstacle_layer, type: "costmap_2d::VoxelLayer"}
    - {name: grid_layer, type: "navigation::GridLayer"}

So to my understanding the static_layer should be the "lowest" layer. Then comes the obstacle_layer handling the input data and then the new grid_layer (according to this paper). So there are two rounds: In one round all layers update the Bounds to their needs. Then in the second round each layer writes to the master costmap (master_grid), right? So now the critical part of the code I don't understand is the following:

void GridLayer::updateCosts(costmap_2d::Costmap2D& master_grid, int min_i, int min_j, int max_i,
                                          int max_j)
{
  if (!enabled_)
    return;

  for (int j = min_j; j < max_j; j++)
  {
    for (int i = min_i; i < max_i; i++)
    {
      int index = getIndex(i, j);
      if (costmap_[index] == NO_INFORMATION)
        continue;
      master_grid.setCost(i, j, costmap_[index]); 
    }
  }
}

In the master_grid I clearly see obstacles moving around since the obstacle layer updates the master_grid as wished for. But then when its the grid_layer's turn to read and write the master_grid, the whole master_grid seems to contain NO_INFORMATION values (except for that tracking dot of course). Am I missing sth? Why does the grid_layer not see the obstacles updated by the obstacle_layer to the master_grid?

Is there an option in the yaml file I need to adjust or is it sth in the code?

Thanks for your time.