villager despawing

I have a spawn light up whole next to my base and made sure no mobs can get in. In side that whole I had about 10 villagers. Randomly they complete disappear even though it is impossible for them to get out. I couldn't have been zombies because only 1 or 2 disappear at a time for no reason, even thought their is no reason for this. The same is also happening with a minecart with hopper that I keep replacing for it to despawn again. I am only in the mid game in get extremely annoyed when it disappear and cost me a lot of iron. And less be honest those thing take forever to craft.