Planes and Volumetric Lighting (Unwanted Semitransparency)

We are using HxVolumetric Lighting for our Game. hitboxteam.com/HxVolumetricLighting/

Our game takes place in a forest and to save performance, the leaves are modelled as trees. This works quite well but the leaf then only has one side because it is a plane.

To fix this, we have created a two-sided shader.

However, Volumetric Lighting does not seem to recognize the plane properly, one side (with the positive normal) is opaque as wanted, the other is semi transparent.

We have tried many different shaders and this seemed to be a problem with all of them.

Are there any alternative shaders (third-party) which we could try? Have we missed an option in HxVolumetric Lighting settings?