It's a bug of the game itself.
There're 3 kinds of cards which may activize the bug: bite bug, red bat and blobra. Sometimes u may find those cards disapear suddenly for no reason while u r walking in the world map.
According to the post I read, it's a good thing if u lose any of those cards, and the system won't have any trouble on u. But if u don't, something weird, or even terrible may happen. The Galbandia battle thing of mine is a good example.
Except for the Galbandia battle, those cards, if u didn't lose them automaticly, might affect things, like the Oble Lake misson.