The Zen Peaks map was the one that messed up recently for me, it did it twice the same night playing with random groups. One match it was plant side, the other time it was Zombie. I am guessing you are right about one spawning below the map or something, since when it happens there's no way to actually find/get to the last one.
what wave is this happening on? does it happen during a specific challenge?
Those two instances happened during a special challenge, the plant side wanted chickens collected, the zombie side wanted candy (?) collected, not sure what exactly since we failed it as nobody could reach the first object in time, so then we went about the usual killing spree and got stuck with a missing kill.
