Me too. I'm relatively certain I know what's doing this. The milestone "Aggressor" auto-completed on a lot of people's accounts, only it didn't count toward "A Tremendous Service". You'll need to play more Starfighter Assault to win on all 5 maps. Not confirmed, because I still haven't won as the attacker on one of the maps yet.
I confirm this, I was stuck at The Agressor at 3/5 because it is a buggy milestone, just like its brother The Defender, though the first one unlocked itself automatically when I was still at 3/5, so I took the credits ofc, but now I will not be able to unlock A Tremendous Service which is stuck at 21/26 even though I only have 4 milestones remaining, including The Defender of course, even though I won at least once as a defender on every map.
