Houdini arnold cloud volume render problem

[spe006 0]

Hi guys

I have a cloud volume render problem. 

I creat a instance VDB cloud and use ass to arnold render

But in some places there appeared undesirable lines

I will not have this error when I test in a single cloud and I only use  standard_volume as the Shading

I am also sure that there are no lights in these directions(2 arnold light)

The horizontal or straight line in the image below is my problem

I don't know how it is caused, is there any solution?

Edited January 10 by spe006