On the " Explore The Ancient Forest" part of the quest, it is possible to get to the dragon without triggering the Roar, and as such not entering the " Follow the sound of the roar in The Ancient Forest". This means players can sometimes kill the dragon and still be stuck on the “Explore The Ancient Forest”.
From what I could gather this bug comes from the fact that the roar triggers only at a certain location, but you are not required to pass that location to get to the dragon.