A NPC wanted my help for clearing a certain areas on level 1 and level 2. But after I cleared the area on level 1 and talked to him, he was glad that I helped him and left. And I checked Dungeon Journal and found this quest is completed.
But I haven't cleared the area(s) on level 2. This quest completed too early. The issue happened on the level 1 of the Underdark dungeon.