MultiVolume Rendering with VTKOpenglGPURaycastiongmapper : rendering failed problem

I’m trying to use vtkMultivolume with vtkOpenglGPUVolumeRayCastMapper in VTK 9.3 in C++

When the blending mode of vtkOpenglGPUVolumeRayCastMapper is set to SetBlendModeToIsoSurface it works very well but the rendering details are very rough, as shown in Figure 1

Because when using vtkvolume with vtkOpenglGPUVolumeRayCastMapper and setting the blending mode at SetBlendModeToMaximumIntensity, I can get very good rendering details, as shown in Figure 2

So I tried to set the blending mode of vtkOpenglGPUVolumeRayCastMapper to SetBlendModeToMaximumIntensity in vtkMultivolume case to get better rendering details, but the rendering is failed. The following is the error code

Did I do something wrong?

Please give me some suggestions.
Thanks