Villagers dissapearing in farms

The villagers in my auto farms are dissapearing randomly. The farm land was all planted and when i come back, it looks like one block is now a dirt block and the villager is gone.

What I expected to happen was...:
the villager should be locked as i had made trades and used nametag on them and it was safe from being attacked

What actually happened was...:
villager is gone

Steps to Reproduce:
1. i do not know how it happened
2. ...
3. ...