I think this is a bug, but it depends on what the author's intentions are.
I am not sure if you wanted floating structures to be a thing, but it can be done by building structures on the floor, building a structure on top of another structure, then deconstructing the initial structure. An example of this is walls. You require a platform, but then after building said platform and walls, you can deconstruct the platform and your walls will float. Is this intended behavior?