Why Can't I paint foliage on my HISM?

I’ve used a spline mesh to spawn hierarchical instanced meshes for cliffs in my level, but for some reason, I can’t paint foliage on the cliffs. There’s a marketplace asset that I bought with HISM buildings and I CAN paint foliage on them. Is there a setting I need to enable so that I can paint foliage on my HISM asset?


When you make the HISM original, set it’s object type to ‘world static’: