OK, try it with a terrainscape material on your terrain, then do the Alpha plane, what do you get now.

Reason I ask is, normal landscape material and the Alpha is how it should be, use the trrrainscpae then the Alpha plane and I get an Alpha plane with terrainscape on it....!!
[Rockyfield]
If I do the plane first, then the terrain it's not an issue, but I would normally do the terrain first...

Injustice will be avenged.
Cofiwch Dryweryn.