particle instances not excluded in alpha mask

I have a pieces of a rendering I am working on. This is the gravel with all the landscape particle instances. If you look at the generated alpha channel mask, you will see that none of the small plants and bushes are excluded in the mask, like the trees are. The tress are group instances. If I render this same mask using B-Maxwell, Maxwell Render for Blender, all of the particle instances are included in the mask. I do this because the client might say make the gravel lighter or darker. If the plants are not excluded in the alpha, they will get changed as well. Not good. I hope this is a bug and can be fixed.

Mask


Gravel with particle instance plants and shrubs