We have the villagers set up in a triangle formation and it is the same two villagers on the one side that keep despawning. Their "cage" is set up the exact same as the other villager that never despawns. we insured that the villagers are safe from the zombie which is in the middle. we have tested it countless times and the zombie CANNOT reach the villagers. We also insured that the villagers have room to move and a bed was set up prior to the zombie being add so that they can sleep in a bed. Lastly we secured them in their "cages" so that they cannot escape. Despite all our efforts the same two villagers keep despawning.