mobs disappeared

every time I go into the world, some mobs will disappear.
 
That was what happened. I place 5 villagers in a safe area (shown in the picture). one day I want to trade with them but I found that there are only 3 villagers left in the village. Than I quit the world and go back again, 2 villagers left this time. Then I try to restarted the app and go back the world again, 1 villager left. At last, all villagers are gone as I am keep trying to restart the app.
 
Also, a number of my feeded cows and sheep are disappeared during this.
 
 I'm so confused that why my mobs will disappear as I login. Please check and fix this problem and thank you for reading this.
 
My phone is Samsung sm j7108