Milk dispensers are unique in that they can't have air over cooling. Sanitation requirement. Basically they are a cold plate design. keeping the door gaskets in good shape and the door lock is important to prevent excess ice. You also have to keep the tube clamp in and always have boxes in all pockets. You don't have to cut the tube off for your stand-by box.
The ice is coming from humidity/moisture in the air. Thats why the door sealing is important. They are not self defrosting. If it builds excess ice in one area of the cold plate, (ie; the back wall) and then spreads, it may be undercharged with refrigerant.