Usually when you discover the entire map of a zone, you get an exploration achievement but it doesn't look that way for The Wandering Isle. I've discovered every inch of the island and no exploration achievement. Boooooo.
Can't only starting zone pandas explore it? That might be their reasoning, but that doesn't make sense since achievements can be account wide now.
Think it's the same as how there's no explore Gilneas achieve. (Ruins of Gilneas, yes, but not Gilneas itself.) Only Worgens can access that, just like only pandas can access Wandering Isle, and that's for a limited time. Even with account-wide achieves, why should you have to create a character just to get an achieve that you might not get anyway (due to eventually leaving the zone and not being able to return)?