Fix for grid projector not filling entire terrain?

I'm using a grid projector from standard assets to display a grid on my terrain. This works fine when all the heightmaps are set to 0. As soon as I raise the terrain, the grid no longer fills the entire terrain. As I change terrain heights, it changes and gets smaller and starts moving depending on the terrain I change.

I know a workaround some people use is a directional light with a cookie, but I'm using my directional light as a sun, which changes with time, so I can't use that as the grid starts to move and warp as the sun moves. I tried a separate directional light but this over exposes the entire scene.

Has anyone ever found a fix for this bug? I've looked at the shader code (Projector/Multiply) but I haven't a clue what I'm looking at.

Happy for alternative ways to achieve this grid to be suggested!