The game resets areas you haven't been to for more than one in game week. Which includes deleting all dropped items, if they are not on permanent shelves. This is not a bug, it is intended to work this way.
If you haven't been to the area where you've dropped the map for more than one week (7 days) in game time, then yes, the item is gone and you have to live with not finishing the quest in question, unless you can get the map again somehow. But I think there are very few quests which make that possible.
The Quest is "old school", so to say. You must be careful and think about the consequences of your actions, because you can fail the quests (and the game) in many ways.
Of course, there is always the "load a previous save" spell, if you make a mistake.