Once an exotic armor piece is in your collections, it becomes a part of the general loot pool and thus becomes entirely RNG. Even in lost sectors where the exotics are narrowed down to a specific slot, it's still RNG on whether or not you get the one you want because it's competing with every other exotic in that same slot. If you want, let's say, Ursa Furiosa on your Titan, it's a 1/11 gamble with every other exotic gauntlet (assuming you have all of those exotics of course). That's assuming an exotic drops at all too. I remember farming for a Raiden Flux last year and it took maybe 180-200 runs for a single one while getting at least three of every other exotic chestpiece (I even counted as many as 6 Ophidia Spathes & 6 Sitxh Coyotes), and that was when you could farm Veles Labyrinth in as little as 2-3 minutes average. Now I'm not saying a bug isn't possible here, but very shitty RNG does seem like a potential factor. Personally, while I haven't farmed the LS yet, the exotic do indeed drop from general sources once acquired, as I got the new Warlock boots again from a world drop.