If all your enemies are taking the same path, is it still the best path for each enemy?
If there is a shorter path and they're still taking the long way around with the rest of your instances, there is something wrong with your events.
If you want them not to take the best path because another instance is already taking that path, you can pathfind for each enemy in sequence. Upon the first one finding a path, create invisible helper objects at each waypoint of that path and add cost to those objects, and regenerate the map. Then pathfind for the next enemy, ect.