Disappearing Villagers

Villagers are disappearing in my world.
When I cure a zombie villager and turn him into a trading villager he disappears after some time, even after saving, also I put a name tag on each villager.
And also there is no zombies killing the villagers because they are completely enclosed in an area that is full of light.

Please fix this problem!

Or suggest what I should do if there is a fix for it.