Disappearing entities

So I have three tamed wolves, a cow farm, and a grid of maps in item frames. Sometimes, when I load the world/die and respawn, they disappear. It's not like I just can't see them, the wolf barking and the cows' mooing sounds are gone, and when I check my map grid, the item frames and the maps inside them are gone. Something very strange, however, is that the game still considers the item frames "there," since I can't place new item frames where they were.