This bug existed since 2014, several people have reported the Mordem Thrasher Champion does not attack in Iron Marches, at Monger’s Sink. Several in-game reports have been made.
Also, the flame thrower event to kill the vines is not functioning normally. The flame thrower that is suppose to be really effective at killing is weak and normal weapons do more damage than the flamethrower. My autoattack does above 350 damage, while the flame thrower does 85 damage.