This thread has been up for a month and I've been trying longer than that. Arenanet...........................? Submitting a ticket now, i'm extremely bored of waiting for a response.
Still haven't received it.I just tried disabling autoloot, I only have one itzel spirit poison, and it was up before boss chest. ..........................................................................................? Arenanet???!
Also not working for me... about the fourth week that I have tried in the same ways as OP. I never got downed and never received it. Buff was always up.
Also wish this could be fixed.. Farmed the achievement over a few hours a few nights ago, since the skin looks great in previews. But when I put it on.. it's rather awful. The effect is practically non-existent on an Asura, also.