Houdini Arnold 6.2.1.0 GPU Volume rendering artefact with visible to camera flag off

[boogayman]

Hello.

Have a little bit of an issue with Arnold GPU and Volume visible to camera flag:

I wanted to render out layer with environment but with volume visibility disabled to camera so only volume shadows are visible on the ground. So I could render volume as a separate render with proper mattes on CPU as matte not supported on GPU yet (really waiting for this feature) and GPU volume rendering is way slower still than CPU despite this latest NanoVDB update. I'm on RTX 3090 btw.

But anyway it turned out looking as a bug or not completely supported feature on GPU. See screenshots. It renders fine on CPU. I did these renders in a simpler scene to show the issue:

screenshot 01: GPU render, visible to Camera Rays on for volume:

screenshot 02: GPU render, visible to Camera Rays off for volume:

screenshot 03: CPU render, visible to Camera Rays off for volume (Proper):

See how CPU renders it properly how I would expect it to look but GPU has this weird looking shape. Almost like it can't produce transparent rays in when visibility to camera is off.

Not sure. Help me out here please. Without this feature I would have to render everything on CPU which gonna extend duration of this project significantly. Unless there's some other way to render out environments on GPU and volumes on CPU but having proper shadows and occlusions.

HtoA 5.6.1.0

Arnold 6.2.1.0

Houdini 18.5.563

Windows 10 19042