VDB with transparent object

[octane_berlin]

I have an object which is temmporarily transparent, controlled by a gradient in the opacity channel of the material.
I animate this 'mask' keyframing the 'Offset V' of the material.



At the same time there are some VDB-Clouds and within those the object is visible, even when it is supposed to be transparent:


Any ideas how to fix this would be highly appreciated!