Bug with landscape holes?

Hi everyone,

I have a question about the landscape holes material.

I want to create some basements for some houses so I figured the way to do that is to make a hole in the landscape and put the building on top of it. So you can walk through the landscape and go in the basement which is under the landscape.

But the hole material is very unprecise around the edges. Sometimes I can stand on where the hole is and not fall through and sometimes I fall though close to the edge before I even get to the hole. Is this normal or is there a way to get it to be more precise or is this a bug?

Thanks in advance. :o