Building self-implodes when cancelling a blueprint

[slyrhyno]

I have a freestanding building that is already constructed. If I go to add a roof, then cancel the blueprint before building, the entire structure collapses. The blueprint for the roof could not have been supporting anything as it hadn't even been partially built, so this seems like a defect. I know structures can tend to rely on the first item built, even when there are other supporting vertical scaffolding, etc, but this is at the top of the structure and not even built.