they left out of fear
Bug (I think?)

they left out of fear because you were making an iron farm and iron farms use iron golems and the villagers are protected by iron golems. Without the iron golems, they would have nothing to protect them from threats.

stayng close to a villager borkes the farm you shuld make it far away and you should be sure that there is no caves close

maybe your base is too far from the farm, so try naming the villagers with name tags.
villagers can't despawn

It's likely you made a mistake with the chamber that separates the villagers from the zombies(remember that baby zombies can go under slabs to attack your villagers)just make sure your villagers are enclosed enough from the zombies(using a slab + trapdoor makes the gap too small for baby zombies)
just note that if you completely close them off from each other the fear mechanic might not work. also make sure to light up the villager area

It's likely you made a mistake with the chamber that separates the villagers from the zombies(remember that baby zombies can go under slabs to attack your villagers)just make sure your villagers are enclosed enough from the zombies(using a slab + trapdoor makes the gap too small for baby zombies)
just note that if you completely close them off from each other the fear mechanic might not work. also make sure to light up the villager area
thanks
I'm trying to make an iron farm (this design https://www.youtube.com/watch?v=JrQuD6xWHlA)
and my villagers keep disappearing! Does anyone know why?