I read that some players are claiming "The Aggressor" sometimes is registered as completed even though it shouldn't be, which could account for the missing challenge. In hindsite I don't recall winning on "Unknown Regions" on attack, so I'm going back to SA to try and confirm if that's the issue. Give it a shot and see if you notice the same. It's a difficult map to win on offense.
Managed to get this to trigger finally. Played a bunch of SA rounds, and eventually won on Kamino which unlocked "The Aggressor". Apparently it was bugged and never fully completed, despite being shown as done.
