Structures spawned in midair have residual blocks underneath them

[puncrafter]

I was hoping to be able to generate floating islands with unconventional materials by using a large enough positive offset. While this does generate the structures in midair, it also comes with this obnoxious layer of ground underneath it that I can't get rid of. I have tried using both surface and underground spawning, to the same result.

Here is an image showing an example.