I have examined the example project, and the issue is that you used the “visibility” tool, without having your material set-up for it. This is designed to cut holes in the landscape, both visibly and in the collision. Without the material being set-up, it ends up only cutting the collision.
If you add a “Landscape Visibility Mask” node to your material, and connect it to the (disabled, but that’s fine) “opacity mask” output, you’ll be able to see your hole 
I’m going to see if I can add some hints/tips to the visibility tool about this, and maybe disable it altogether if you don’t have a Visibility Mask node in your material.