Named and non named mobs gone when logging into a world

This has happened to me 3 times when opening up my solo world. I will log off with a pen full of cows with a named horse tied up next to them and a named dog sitting inside next to my bed. But when I log in again the next day, all of the cows have disappeared along with my horse and dog.

 

It doesn't happen every single time I open up my world but since this is the 3rd time in the pass 4 days, I just gotta let it be known.