expedition (runic) monster are immune to poison cloud (gas arrow)

kinda game breaking.

they do get the poison stacks, but dont take any or no noticable damage from it.
im at 10k dps per poison stack - my autoshot does 12k and the normal mobs die within 3 shots.

i literally have no way to kill the bosses there now :P

please check

they do NOT have the "immune to chaos damage" trait...
i picked that by accident yesterday and was lucky with 3 fire rares. not this time, though :(

im playing on normal league and normal monster so far take damage normally.

update: it happened again, but doesnt always. So far i dont see a pattern when it happens.