So for the first time I was able to do my quest in Orban Glades before cleansing the area. I thought i might be bug but was not sure. After i completed my quest the pestilence spread took over but it did not respawn pestilence monster in the area i cleared due to my quest. I still need to clear 60% pestilence monster but there is none left. Feel bad that I just burn a lot of gear to reach difficulty 3 for no reason.
Note: They did appear again after two hours.