Hi Bob,
I've been looking at your file and I think I know what's going on.
The shaders in the terrain-section of the network ends nicely with a compute terrain.
After that you're adding several surface layers, like the strata and outcrops layer.
When adding populations it's often required to add an extra compute terrain node when terrain displacing shaders, like the strata and outcrops shader, are used after the compute terrain shader in the terrain-section of your network. Hope you can follow me
I've included the adjusted tgd-file so you can take a look at it.
I mentioned the distribution did change a little and maybe the terrain also did.
I didn't render it full size (used your crop settings) so I'm not sure.
Hope this will help you a little...
Martin