I am also having trouble with the Mordremoth fight, as the rifts are either appearing off the platform so as to be unreachable, or I cannot close them to finish a fight with a blighted NPC (I channel the close and the rift goes boom, but the fight does not progress). In addition, the blighted NPCs will sometimes teleport around the map; don’t know if this is intentional or not.
Also had problems with the Blighted Canach fight, with him becoming impossible to get below about 20% health or so. I figured I didn’t understand the mechanic, but since someone else brought it up, maybe it’s a bug too.