Whilst it doesn't impact on the gameplay at all, I have no idea why this annoying achievement was put into the game.
Before I took a break, I was stuck on 99.53% regions discovered, came back and started a new play through and now stuck on 99.53% again. I have no qualms being stuck on "Git Gud" achievements or ones I may never even get due to them being above and beyond my gaming skill set, but this achievement is an exercise in frustration for some and requires some folk to sit and scour an entire map for hours sometimes looking for a little island or dot on the map that could be that golden egg. I could be sitting watching paint dry in my living room rather than doing this. I've looked at what feels like all the posts strewn over the net for possible locations and still no dice. Come on, Ubi, at least make seeing these speckles at little bit more user friendly. After all, games are suppose to be fun as is aiming for certain achievements. This ain't fun, it's needless.
No idea if it's glitched. I suspect it isn't. What they should do, is put a discovered percentage in each area description (the part that tells you whether or not you have discovered the leader, the faction etc). If they did this, it would allow us to narrow the search down to a specific region. I mean, it could be The Obsidian Islands and that would have saved me hours wasting my life looking at the likes of Makedonia etc.