Hello.
I had the same problem when working on Twisted Minds demo. I figured out the problem came out from the material used on the vegetation model. Make sure to use shaders from Shaders\Vegetation for Vegetation and Veg Shadow shaders. Also use a shadow shader (shadow+alphamask.shader is the exemple from the doc.)
You can find some documentation here https://www.leadwerks.com/learn?page=Tutorials_Editor_Terrain#section10.1 about vegetation material.
Hope it helps.
Ttiki's post in Terrain Strange Behavior with vegetation was marked as the answer