On events that involve lava being dumped on an area (seen this happen both on the Dredge Fractal at the end boss, as well as the boss of Sorrow’s Embrace story mode), the first time the lava is dumped it shows up properly. For all subsequent ‘dumps’, the lava does now show up and gives no visible indication of its effect. Players still take damage if they’re in the area, but there is no way of seeing where and when the lava is coming down. This seems to last until you leave the zone. Both times I saw this happen in both the abovementioned examples, every person in my group was seeing the same thing.