Every time you use a Map Kit, it disappears.
It makes the map just fine, mind you.
After discussing this with some other players OOCly, the general concesus was that it either:
A) Is probably in relation to that bug that makes folks' gold disappear if they place it on the ground, as one has to use the map on the ground in order to make it work.
B) That the map kit disappearing was intentional, so as to make cartography all the more valuable.
If the case is B, I suggest the following: Instead of making it so players have to buy 10 map kits every time they want to do some mapping out of areas, we instead revert back to the old system where a map kit lasts forever, but also make it a lot more expensive. It's just a pain in the neck at the moment.