The editor copies the "terrain.data" file in several different locations. The "original" version that is open by the editor and copied everywhere else is here:
\Divinity - Original Sin\Data\Editor\Mods\ModName\Levels\LevelName\Terrains
You should NOT edit the copy that is here:
\Divinity - Original Sin\Data\Mods\ModName\Levels\LevelName\Terrains
Ugh, thanks, completely missed that little detail in the tool.