To be honest, stacking itself isn't even needed. What IS needed is the ability to hold more than one at a time. There's no logical reason whatsoever that an untradable item that only you can use should be a unique item in this way. The other maps, fair enough, as they can be traded.

The most valuable maps in the game, unhidden maps, can stack and aren't unique to prevent them being lost if they drop. Well mysterious maps should behave exactly the same way, especially as we can easily get several a day from the quest and due to the soldiery drop rate.