The API - ccbUpdateTerrain had a bug and now is fixed. Plants on a terrain loaded from heightmap now show as expected. Thanks for letting me know.
ccbSetTerrainSmoothing is an old API that I never ended up using so ignore. It was meant and partly did work to readjust the angle of terrain angles to smooth out sharp changes from low resolution heightmap file. It worked sort but not great so dont worry about this. You can smooth out sharp changes in photo editing software instead and works better IMO.