Apparently, Disillusioned Adventurers Will Save the World, Vol. 3 (manga)

A band of reject adventurers who were all kicked out of their original parties (and have now been deemed a practically ideal team by the adventurers guild for some reason) sets out for a labyrinth to find an ancient artifact called the Sword of Bonds. The gang breezes through the newbie-friendly labyrinth with hardly a scratch�_"when suddenly a mysterious passageway not shown on the map appears before them...