This year, the problem is apparently that certain trick or treat buckets in Northrend are incorrectly flagging buckets elsewhere in the world as already looted. If you need the candy bucket achievement (because you did not complete it last year), and you loot the buckets in the "wrong" order, you will be prevented from completing it. Blizzard says they're working on it.
It's not quite the generic "content failed to reset from previous years" issue that has come up repeatedly, but it makes the case yet again that there are serious problems with tying a long-term achievement grind to time-limited events that change just enough every year to make sufficient scale testing nigh impossible.
On the plus side, this is one of those things that is no longer my problem.