It's a limitation that we're aware of, and it occurs whenever the camera passes through the volume prescribed for the clouds. Here's a possible workaround, though. I've found that if you use the "localise" setting you can have clouds that the camera doesn't pass through, and use those to build clouds all the way to the horizon without running into this problem. You might only need two clouds layers to do this; distant clouds and foreground/midground clouds.
I'll try to get this fixed for the next version.
Matt
