Villagers despawn in my world when re-loading the chunk.

It happened twice now: 

1st time, I went to the nether. I used the nether to reach one of my overworld farms which was 1000+ block away from my villagers. Coming back through the nether, I noticed that one of my villagers disappeared.

2nd time I went to one of my mob farms which was 200/300 blocks away. When I returned, one of my villager dissapeared.

In both cases, the villagers were nametagged and had their profession locked.

Both villagers had a workstation and bed.