You don't have to re-kill the specific dragon that didn't register. You just have to kill any of the previous dragons.
As things currently stand, you could kill a dragon like the western approach dragon 10 times and get the achievement, even if you don't kill any of the other dragons. Crestwood Dragon is the only one that I've seen mention of that doesn't grant the achievement if you go back and kill it again.
Also, I'm not denying that it's a bug. It is a bug and it's an annoying one at that. But the solution to get the achievement is simple.