Guild Wars 2

I had noticed that I had 98% completion in the Wayfarer Foothills area, so I decided to get my last PoI for completion. Upon 100%, I did not receive the rewards chest for area completion.

… because you already got it once before? There was a new POI added.

Khisanth is most probably right. You cannot get a reward twice, so if you already completed the map before they added more waypoints and PoIs you won’t get another reward. Your map % goes down once they update and add new content in it.