We discovered the problem when resolving T15426. When reproducing the steps described in this bug, the internal pixel type of the image seems to change.
When checking GetInput()->GetNumberOfScalarComponents() in the vtkMitkOpenGLVolumeTextureMapper3D::IsRenderSupported() method, it seems to change from 1 to 4 in the second initialization step of the activating the "GPU Slicing" volumerender method.