"The End?" achievement fails to unlock

I have entered the End and slain the enderdragon, but I did not get awarded "The End?" achievement for entering the End, nor did I get "The End." achievement for killing the dragon (since the prerequisite didn't unlock).

I have since entered the End a few more times (through the same portal), but the achievement remains locked. I have completed all prerequisite achievements, and am using an unmodded version of Minecraft.