FF14 has a quirk in their achievements in that if you already meet requirements prior to the addition of the achievements you won't get the achievement until you update whatever the requirement was.

Example; if you had 220 triple triad cards before they added the achievement for it, you would not get the achievement until you got another card.

If you had 25000 pvp wins and a 25000 PvP win achievement is made, you would not get it until you have 25001 wins

My point is that if you complete all quests in an area, there's be no way to be given the achievement because there'll be no way to update the flag that you've done all the quests.