Hello all,
Just to add an additional piece of information that may help someone in the future.
I faced the same problem that dais davy reported when I modified an original terrain under the methodology of including some modifications by creating a fake geometry, exporting it as a terrain and later combine the original terrain with the new one containing the modifications.
I solved it by using the option of right-click on the new terrain that combines the original one and the modificaitons\Generate New RAS Terrain\Resample to single terrain
I hope this may help.
Javier