My villagers keep despawning.

I have 3 villagers in an iron farm in my minecraft server. It is not a server issue, as this has happening in multiple other worlds. I have even nametagged my villagers, but regardless of what I do, I'll come back to it sometimes and anywhere from 1 to all 3 of them have despawned. They are not getting killed as they are completely enclosed except for a single block on the side.