This post is super old, but I just encountered this issue trying to use the Lush Plants megascans pack off the Unreal Marketplace (and also some test meshes I made in Maya).
The mesh is probably missing vertex colors. If you bring the mesh into, say, Maya, and go to Mesh Display > Toggle Display Colors Attribute, the mesh should be painted in red and green (with red being the areas that deform more from the simplegrasswind material). You could probably paint this in on custom meshes.
If it has that, then you just need to reimport the mesh into Unreal, and set the Vertex Color Import Option to Replace (The default setting is Ignore).
Hope this helps someone.