So they are taking on a dungeon where adventurers already managed to reach the 4-5th floor; even if they don't have the map of the region they would have the list of regular monsters encountered there and should know right away about the mushroom floor/forest zone.
Unless the dungeon is ever changing the zones and types of monsters, but then wouldn't the same apply to the first three floors?
Bah this isn't making a whole lot logical sense.