Shockingly, having come here to be frustrated about things. I'm now one of the few accounts that's been successful with 2 of the Sandswept Isles rifts... the 2 that start nearest to [Atholma Waypoint] (one is in the bay, one is at the tip of the The Iron Whale). So I've finished the achievement, but with a different set of successful rifts to others, perhaps it is account bound after all? And just to help frustrate others, I had completed both of these multiple times before the one I've just done was eventually successful. So, maybe just keep trying guys? :(