Cured villagers despawn

I couldn't find a village near my house, so i decided to cure zombie villagers instead. after curing 2 zombie villagers, i bred them a little so i could have a villager trading system. after i went on an adventure to find the stronghold, the villagers despawned. just the ones i cured. i thought it would be a once-off, but when i went to add a nether portal to my stronghold, they despawned again!

If it helps, i started playing on peaceful(mostly because of the nether now not being as safe) just before this started happening.