Edit: Duplicate (sry can´t delete) => 

*https://bugs.mojang.com/browse/MC-144784?jql=project%20%3D%20MC%20AND%20resolution%20%3D%20Unresolved%20ORDER%20BY%20updated%20DESC*

All Entities or Mobs can disappear upon going away too far and/or dying or reloding the save state. 

Example: I had a village with an Iron golem and a big Farm with sheep, cattle, pigs and chicken. After I went looking for a stronghold and came back all of them were gone. Reloading the save state didn´t help. Reloading the game the other day made them appear again.

Edit: I have included now screenshots from another farm where I have encountered this problem. A save game reload has solved the error for this one.