Fixing grass mesh shader for Hight terrain addon

Godot Version

4.3

Question

Hello, recently I have been painting 3d grass mesh onto my terrain I made using the height map terrain addon. My problem is when I paint the mesh on to the terrain, then add a shader to the mesh, all the grass gets sent to y level 0 instead of on the surface of the terrain. I was wondering if anyone knows how to stop this from happening? Thanks