All the mobs that are in 50×50 blocks got disappeared for no reason.

I've lastplay my world on yesterday when today has come I've decided to do some project but when I load up my world. All of my mobs(Animals,Villagers,My tamed dog and cat,Armor stand,My trident that I have thrown my rarest Zombie with diamonds armors) are gone! but luckily all my item didn't disappear. And i have noticed that all of that mobs will got disappeared in the 50×50 blocks area(From player center) my village that far away from 150blocks up didn't get disappeared. So please fix this bug.. this is very important.