When an opponent plays Cartographer, there's an empty selection box for which cards to discard or put back.
That's intentional to visualize that your opponent does a selection from cards you cannot see.
While it is true that the box is intentional, it is not intentionally empty.
There will be card backs showing in this box at some point in the future.
Oops, sorry, that's even better. :)
I've noticed the same thing with Sentry, I believe.