During a normal horde night the game will spawn a horde group and send them to wherever you are. The game also will send any zombies that are already alive in the area to your location as part of the horde night event. The 21 day setup can only stop the horde group from being spawned so you will still get any zombies that are already alive in your area sent to you by the game. So, for example, if your game stage would normally have a horde group spawn size of 50 and you have 10 zombies already alive in the area then instead of getting 60 zombies you'll just see the 10 that are already alive show up to try to kill you. This means that if you want as few zombies to deal with as possible on a non-horde horde night then make sure that during the day you are killing anything that's alive around you to keep the numbers down.
This is a bug with the base game and can affect any mod. There currently isn't anything that the modders can do to work around this. The only current work around is for players to always eat from their toolbelt for a guaranteed proper use of the item. The same thing can sometimes happen when reading books or schematics from inside a bookcase. You can normally put the stuff into your bag and use it from your bag but there is still a chance something will not actually work properly from within the bag. You can normally reduce that fail chance by not moving or doing anything while using an item from within your bag.
