If it’s of any help to anyone (hopefully the devs investigating this stuff), I’m working on the same meta-achievement and did ‘Avenge Me!’ on 20th April this year. That makes me think that this might be another bug introduced when 10.2.7 was released on May 7th.
It might even be causally linked to the other achievement-related bugs mentioned in this thread and elsewhere.
@Mutio: you might also be interested in this post that mentions your problem.