This is due to the fact that the duality farm is a much easier task and a server end change rather than an update or a fix. It seems there are changes that bungie can easily implement on their end that don't require full server change or client side updates. In duality it is as simple as removing a checkpoint and changing the drops so that they allow for low end armor. The ultimate fix will be for Ghalran to not fall off the map, like many other bosses in the game, but that will be a full update that will require more time to implement. The Invisible guardians isssue is more of a client side issue, based off the fact that resetting the game/console fixes the issue temporarily, and therefore requires a update to both the servers as well as how it runs on individual consoles/pc's. I know that it seems as though Bungie is targeting things that are "beneficial" for players, but they generally fix things as they can and this was just an easy exploit to quickly make a work around for the moment.