I don't think it's intentional, but a darkness spell (Casted by a Relic Guardian, 2ed slot normal, chaos, retribution domains) seems to make pcs or enemies invisible and blind within it. Was working fine a few days ago, to my knowledge, and seems to work fine half of the time if not used near pcs/npcs.
Thanks for the report, tried to fix. Let me know if it's still broken.