There was something similar with another holiday that I recall…
I’m pretty sure it was Hallow’s End. Someone had the helm in their bank, but it wouldn’t complete the achievement. The distinction was that you had to acquire the helm once the achievements went live for it to flag as completed. Not to already have had the piece.
So it may just be something along those lines again. Please let us know if doing it all this year did the trick.
[EDIT] Yeah, I thought I remembered that being an issue with Hallows End. Again - this may not be the true case for your particular issue, as you’re the first person to come here with this. It’s just some precedence that it was an issue for another holiday since achievements went live with the release of WotLKC.
