Basically its just what it says, baby bunnies dissapear from view getting not far from them, however seems that big bunnies can be seen from quite far as usual. I havent tested but it may be the case with other mobs or their baby forms as well. I added two screenshots.
Bunny in further photo is the same adult bunny as in first one. but baby bunny doesnt show up even though he is here. I even got back and forth to make sure baby bunny is here and yes it reappears as soon as i get closer again.