This has been broken for some time now. In Halo 3 Forge, when you set an object to Not appear at the start of the round, the option will sometimes not work, no matter how long you set the timer. For the Kill Ball, it simply doesn’t follow that rule in Forge Mode. It works in Gameplay however. Though for certain forge techniques, it makes things needlessly more difficult.
Another addition to this bug: This problem happens with Any immovable object in Halo 3 Forge. Also, If you Save a map with an item’s “Place at Start” function set to “No”, when you load up the forge map again, you Cannot select or edit the object’s options.