I've only tried casting it once so I will try again, but the darkness seemed to glitch out and didn't give the effects of darkness, just a patch of blackness on the tiles cut off with a line
It's not specific to Phantoms of Deception but to all darkness vfx, and this is a thing since the latest NWN version 8193.13.
2 of the DMs have already submitted feedback to Beamdog about that, and I encourage everyone else to do the same.