So, what does the FOOD_DUMMY oe data look like? How much life do the units have? Is there an AoE damaging spell in that region that could be killing them? etc.
Do you have a trigger that fires when a unit enters Center of food dummy place <gen>?
Also you might need to only be adding the expiration timer to the last created unit, because every time another unit enters 'teleport north units' , you would be adding an additional expiration timer to every unit of that type that is still alive. The problem with that is, you are creating multiple units at once - which means you might want to use a for loop.