Igloo basements not spawning, even with trapdoor.

An Igloo I have found and multiple igloos I have seen spawn with the trapdoor, which should lead to the basement, but simply don't. I know not all igloos have basements. But some of them have trapdoors indicating there is a basement, but there is really not one. It looks like this is the case because the igloos are constantly found floating above the ground.