OK, it was partly operator error.
The network uses two merged fs shaders. I inadvertently set the density variation scale of the smaller size stones to zero. (Probably meant to set vary density to zero.) This resulted in some large gaps where the smaller stones were missing.
However, it appears that version 4.6.31 of Terragen handles this setting differently than earlier versions. So when I use it to render the scene, the smaller fake stone layer appeared to be fine even though it really wasn't. (This why I'm only taking part of the blame
).
When density variation scale is set to zero, there are serious mismatches between fake stone displacement and color in both 4.5.71 and 4.6.31. But the displacements are inverted.
Lesson learned: Pay very close attention to the density variation settings. (I'll be disabling this feature entirely for this scene.)