I got one-shotted 2-3 times today in the Vault of Umbrage on the venom caller green screen, not being anywhere near the poison flower AOEs.
As I'm not a Maelstrom expert, I assumed I had not quite fully understood the mechanics, however after defeating the Behemoth, I walked towards the champion's chest quite satisfied and got one-shotted again, although the fight had ended a few seconds back.
The attached screenshot shows the chest with the two NPCs, clearly proving the fight was indeed over when I got this surprise one-shot.