Villagers despawning

While in a Minecraft 1.13 server (pre 1-5), I have noticed the disappearance of villagers in my iron farm. It should be noted that the villagers are surrounded by all sides, and recognize the farm as a village (causing iron golems to spawn).

I only noticed the disappearance after a few hours.

It should be noted that I experienced the issue in pre 3 and pre 5, but I have not tested this on other versions.