Explorr Steam Achievemen Bugged?

[Hayashi.226]

Playing on Steam, I've just arrived in Hearthsong, and had this achievement unlock. I haven't gone any further beyond Hearthsong, so I've yet to "visit every map."

[Ryan Morris]

Same thing happened to me when I played through the game. I thought it was a bit odd too. I only had about 4-5 other zones to discover beyond that point so it was basically everything for me early.