People seem to have an impression that there is a large number of developers behind this game and they can be thrown into a specific issue to fix it quicker. In a fact, the number of developers is actually very small and the number of active developers even smaller. What you see is actually a work of a small group of developers, who have somehow managed to make a great game.
I haven't been around for a while, but I believe there is only one guy who knows how the NPC client code works and there aren't any other resources available to put on that issue. Asking four prospect coders to work on the NPC client doesn't solve the problem any faster, perhaps the opposite.
This is a hobby project and people work on it only after they have done with all their real life work, families and everything else including playing some games. New developers come from the current player base and if you are a software developer, take a look at the sources and perhaps you are the extra resource that can be put on this issue
Without knowing the details, I guess the current issue is the change that made NPCs impervious when they get stuck to avoid people to lead NPCs to places where they can be easily killed. Might be that the code is too sensitive and makes NPCs impervious a little bit too early.