The animals keeping on disappear

Hello I just start this game and I create a world with the setting of survival-normal, and i pet some cows, chickens and sheeps however they are keep disappearing and they will disappear at the same time, and already happen for several times. The picture below is the place that I pet them, as I first start this type of games I am not sure did I pet them correctly, can you please take a look or if this is a bug? Thankyou.