Triplanar Mapping on landscape issue

Hey Guys,
I tried to build a triplanar mapping for my landscape material. But if I use the “Flatten” modifiere, the triplanar mapping is not working. Do you have an idea why or how to fix it?
I followed this tutorial for the triplanar mapping:

On the left side of the picture is a mountain which is normaly sculped, the right one is sculped with “Flatten”

I have the same thing in my terrain setup but I am going for more natural looking terrain so as long as you don’t use just flatten but put a small amount of falloff onto the brush you should be fine.

This is just how it works.

Okay, thanks