It was meant to work like this in original H2, according to the in-game description (known land or some similar terminology). And it is good for balance too, as you can't use it to speedily uncover terrain, but you can use it in discovered lands. What do you think about preventing DD from targetting any area that is undiscovered? This is the way it is in fheroes2, and it seems to work well.
What do you think?