During the beta there were a number of other achievements that would not be awarded, and the developers fixed all of the reported ones.  I'm not sure if anyone ever complained about this one, though.  I know I, for one, never had enough time to get all the way through the scenario.

Hopefully the developers will see your post and fix it like they did the others.

Perhaps that's true.  During the beta I got all the monster-killing achievements except Torax, and I did them in random maps before trying the scenario.