vtkMaskPoints does not passes through cell data
*vtkMaskPoints is a filter that passes through points and point attributes from input dataset. (Other geometry is not passed through.) *
Emphasis added. When it comes to a mesh that is a VTK_VERTEX, there can be either point or cell data. It does not matter as they are the same. Regardless both point and cell data should be passed through. The vtkMaskPoints does not pass through cell data. It should.