Hi,
There seems to exist a problem with the material ID channel using Mix materials.
The problem is that unlike with a single material, when using a mix material (with 50% mix of 2 materials with opacity channels) the corresponding material ID channel is 100% opaque. This is a problem when dealing with, for instance leaves on a tree, or just about any placard based render with the material colour multipass.
In fear I have not adequately explained the problem, I have attached a graphic to show exactly what I am talking about...
Kind Regards.