As dnautics alludes in a sibling commment, the effect you're talking about is subtly floorplan dependent, and that's going to vary from manufacturer to manufact

As dnautics alludes in a sibling commment, the effect you're talking about is subtly floorplan dependent, and that's going to vary from manufacturer to manufacturer. I don't think it's completely impossible that there's a common non-idealness to FPGAs, but I also don't think that the common non-idealness is going to involve using disconnected cells as some kind of antenna.