BSP sections disappear

I have a BSP room, a BSP hollow cone, and a BSP hollow cylinder with half its faces deleted. When working with that cylinder, something wonky happened. Here is a picture of my parts before the cylinder is included (or when it is cut from the scene):

And here after adding the cylinder:

When the cylinder is in the scene, the cone loses its bottom half and the room’s floor (and a wall, barely seen at this angle) disappear. I also had issues earlier where I could not add another cube to my room; inside the room’s cube, it would disappear. Any thoughts why?

Just in case, I have copied and pasted, restarted the editor and the computer, tried in a new level, and (with regard to the cube mentioned in the last bit) triple checked additive versus subtractive.