Collision logic for ghosts

[PFQNiet]

I have a floor-type entity, that other entities can be built on top of. Or you can build floor underneath existing buildings, but that's not important.

This all works fine for actual entities, but when dealing with ghosts it gets a bit odd. Basically if I copy-paste the build then the ghosts are all there just fine. I then start placing the floor entities and sometimes the building entity ghosts disappear, but usually not. I can't seem to figure out any particular pattern to this but it does tend to happen to buildings in a single row of a factory build, or maybe two rows, but not other rows.

I'm assuming that if an entity is built that collides with the ghost then the ghost is removed, but what exactly is the collision logic for it to be so inconsistent?