System Information
Operating system: Windows 10
Graphics card: GeForce GTX 950 OC

Blender Version
Blender 2.80 cc73d59ad580

Short description of error
First of all :
Not sure if this is a bug or if Im just not getting it.

Everything is done in Cycles

Im subtacting the DiffInd passes in compositing and I get the volxel shape of my smoke sim.

Here a screenshot of my Nodesetup (note the backdrop)

I do this to extract the Indirect lighting of for example fire or something else while excluding the sun and stuff.

This is the image (the white cuby thing to the right of the smoke)

Exact steps for others to reproduce the error
Add a basic smoke simm with high resolution and a ground and wall.
Add a sun light
Add a new view layer
Exclude the smoke domain from the 2nd layer
Enable the DiffInd pass for both layeres

In compositing :
Subtract the DiffInd passes from each other.

This is an example scene:

It seems like there is a problem how "null" surfaces get handeled.
If I set a layer to indirect only the indirect light only appear outside the voxels (where they are intersecting the geometry).