In assist mode it will not fight Irminsel in the Void Ark, its as if it no longer recognizes it as a target to attack.
If you select it it just autoattacks. (using monk profile)
Assist works fine on all other mobs there.
it's not just the mnk profile i can tell you that much and also i think it's more along the lines of Line Of Sight issue, due to the way it happens in other dungeons such as The Aetherochemical Research Facility when you go to fight the balls that are kind of in mini pyramids then same thing happens, although can be countered in that map by climbing to the top of the pyramid, as to why it's not detecting Irminsel, good question but my guess is that since it's a pillar maybe when the target location is calculated it's showing up to minion as if it were underground slighty. (just my two cents)