It’s been known for a long time, but sort of vanished off the radar for much of this century. I can only imagine it’s because it has applications as “mixture separators” as this webpage shows https://www.lanl.gov/org/ddste/aldps/materials-physics-appli... and so it could be used to enrich uranium (in the form of uranium hexaflouride gas), in a more robust way than the stupid legacy centrifuge technology. Thus all the interesting research on it must have been classified.