vtkRenderLargeImage in Python gives wrong field of view in VTK 8.2+

Using vtkRenderLargeImage with magnification > 1 gives images where the field of view is unexpectedly cropped in VTK versions 8.2 onwards. I'm using Python, I don't know whether this is Python specific. Same hevaiour under Windows and Linux. I think this behaviour started in 8.2.0.

Here are 2 images created from the same renderer+geometry+camera using vtkRenderLargeImage in VTK 8.1.2, one with vtkRenderLargeImage.setMagnification(1) and one with vtkRenderLargeImage.setMagnification(2). As expected the sphere looks the same size in both images, but the one with magnification = 2 has twice the pixel dimensions: Here is the result of the same code using VTK 9.0.1: with mganification > 1 the image looks unexpectedly cut-off at the edges (field of view is a bit smaller):

Code to reproduce (uses matplotlib to display the results):

"""
Testing of vtkRenderLargeImage at different magnifications
with off-screen rendering.

Expected correct result:
Two images of a sphere which look identical size,
but one image has twice the pixel dimensions as the other.
"""
import vtk
from vtk.util.numpy_support import vtk_to_numpy
import matplotlib.pyplot as plt

if __name__ == '__main__':

    # Set up a small off-screen render window
    renderWindow = vtk.vtkRenderWindow()
    renderWindow.OffScreenRenderingOn()
    renderer = vtk.vtkRenderer()
    renderWindow.AddRenderer(renderer)
    renderWindow.SetSize(300,300)

    # vtkRenderLargeImage to be tested.
    vtk_win_im = vtk.vtkRenderLargeImage()
    vtk_win_im.SetInput(renderer)

    # Create a sphere and add it to the renderer
    sphereSource = vtk.vtkSphereSource()
    sphereSource.SetCenter(0.0, 0.0, 0.0)
    sphereSource.SetRadius(3.0)
    sphereSource.SetPhiResolution(100)
    sphereSource.SetThetaResolution(100)
    mapper = vtk.vtkPolyDataMapper()
    mapper.SetInputConnection(sphereSource.GetOutputPort())
    actor = vtk.vtkActor()
    actor.SetMapper(mapper)
    renderer.AddActor(actor)

    # Camera Setup so the sphere just touches the edges of the field of view
    cam = renderer.GetActiveCamera()
    cam.SetPosition(0.,0.,5.)
    cam.SetFocalPoint(0.,0.,0.)
    cam.SetViewAngle(73.74)

    # Render image with magnification = 1
    vtk_win_im.SetMagnification(1)
    renderWindow.Render()
    vtk_win_im.Update()
    vtk_image = vtk_win_im.GetOutput()
    vtk_array = vtk_image.GetPointData().GetScalars()
    dims = vtk_image.GetDimensions()
    im_mag1 = vtk_to_numpy(vtk_array).reshape(dims[1], dims[0], 3)

    # Render image with magnification = 2
    vtk_win_im.SetMagnification(2)
    renderWindow.Render()
    vtk_win_im.Update()
    vtk_image = vtk_win_im.GetOutput()
    vtk_array = vtk_image.GetPointData().GetScalars()
    dims = vtk_image.GetDimensions()
    im_mag2 = vtk_to_numpy(vtk_array).reshape(dims[1], dims[0], 3)

    # Display the images; they should look the same but one at higher pixel count.
    plt.subplot(121)
    plt.imshow(im_mag1)
    plt.title('Magnification = 1')
    plt.subplot(122)
    plt.imshow(im_mag2)
    plt.title('Magnification = 2')
    plt.suptitle('vtkRenderLargeImage test: VTK {:}'.format(vtk.vtkVersion.GetVTKVersion()))
    plt.show()