tried to get this to work on multiple toons today with the Spicy Nexu Supreme mission and could not seem to progress on the achievement at all. Every time I checked the achievement, I was stuck at 27/31. I tried on multiple toons both impside and pubside.
At the suggestion of a guildie, I went back and tried the non-challenge dish and it popped. So, if anyone is having an issue, I suggest trying that.