I am trying to make a village, but my villagers keep despawning.

I cure a zombie villager, then I put it in a 100% mob proof box. I’ve tried everything, 

I’ve named the villager, put one in a vehicle, I’ve even equipped a zombie villager with armor and cured it but still, my villagers keep despawning as if they were just any old mob.

i would like to note that I found an abandoned village with zombie villagers in it and they did not despawn.

that reminds me that named mobs also are despawning.