Probably because Reforged uses different filepath for terrain types. I believe they were in _hd.w3mod/terrainart folder rather than /terrainart. I personally never worked with Reforged textures
@Retera might know something about it. I believe you can use manually input those terraintypes not sure how it would look without orm and diffuse ones though.
Edit: Oh wait you can't because it takes integer as an argument.
Well a solution would be creating image instead of terrain to replicate the functionality.