First user transform ignored by vtkMultiVolume
The user transform set to the volume actor at the 0th port of vtkMultiVolume is not taken into account, and an identity transform is used regardless the user transform.
The issue came up within 3D Slicer (branch https://github.com/cpinter/Slicer/tree/multi-volume-rendering), and another developer was able to reproduce it too, see http://vtk.1045678.n5.nabble.com/vtkMultiVolume-and-shading-td5747172.html#a5747460
However when I tried reproducing this in the test https://gitlab.kitware.com/vtk/vtk/blob/master/Rendering/Volume/Testing/Cxx/TestGPURayCastMultiVolumeOverlapping.cxx, I wasn't able to, so some condition must be required for this bug to come out.