Matte Fog and Cutout Opacity in Daz studio 4.20

Hey guys. I loaded a plane into the scene and applied 2 textures to the surface. A diffuse map and a opacity map.
I needed to cover the void in the scene. I did this in version 4.14 and had no problem with it. I also use the "Matte fog" option enabled.
This creates a sense of airy perspective. But in version 4.20 I see a distinctly "transparent" area. 
As I wrote above. There was no such problem in version 4.14. It seems that nvidia made some changes to iray. Any idea how to get around this problem?
I tried using Weight Refraction instead of cutout opacity it didn't help. 

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