I have come across this in 14w11b, there is somewhat of a work around though: This only effects the last map in the stack. So if you have two maps, the first one you click will register with the scoreboard, but the last will not. Still a bug that needs fixing though.
Still in 1.8.7 , and I just wanted to note, that the objective changes if you have at least 2 empty maps in one slot, then open one and the number increases by 1.
Someone ?