Canthan Missions Problem

So, I finally got Masters on all 13 missions.

But the game persists that there's 1 I haven't done. I've gone over the map loads of times, and every Mission shows that I've got Masters on it, yet my title progress bar still says 12/13.

Is there something I'm missing?

Possibly, you are missing either Gyala hatchery or Eternal grove. You can finish the gyme by only doing one of those and never travelling into the lands of the opposite faction, so you wouldn't see the outpost.

Gyala Hatchery is a luxon quest though. I remember doing all the Kurzick stuff, so how would I get to do the Luxon quest then?

EDIT:

Nevermind, apparently Jamei in Maatu keep is leading me to Cavalon, where I've found the Befriending The Luxons quest.

I'm assuming this will help me get to the Mission I need to complete if I just follow these quests, is that right?

Indeed. Jamei's quest will let you get to Cavalon, where you'll get the Befriending the Luxons quest (gain 10000 faction). After you do it, the next quest will take you to Gyala hatchery outpost.

You can also backtrack from Unwaking Waters explorable.

Harvest Temple -> Unwaking Waters (explorable) -> Unwaking Waters (Luxon) -> Silent Surf -> Leviathan Pits -> Gyala Hatchery (explorable) -> Gyala Hatchery.