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What do you mean exactly by bug you should explain in more detail.
I have completed the achievement with no problem after faction changing to Alliance and back.
The only catch is that it resets all neutral quests making you do some extra work if you already made some progress in certain zones.
However if you have done X faction specific quests lets say Horde side and faction change twice (Ally and back), all those quests will reset as well, which is a huge setback.
That's my understanding of it, and its the closest I could think of when you say 'bug' since I don't recall any other problems ever in the past.
Sorry for not explaining. As you know all the achievements in the interface look like this:
https://i.imgur.com/YXVTPvL.png
The bug I had seen in the bugtracker months ago consisted in that you had all the check marks ✓ in green (i.e. you had completed all the continents) but the achievement itself was greyed out.