Entities tend to partially despawn when the player roams to far away from them. I say partially because the game still treats them as there, they just become invisible and their hitbox disappears. I assume this is because the game does not render them when outside of a particular range, and then fails to "re-render" them when within range again. This seems to happen at particular, yet random locations and the glitch seems to be chunk based.

One thing for sure is that the game will render them again when said entity is teleported to a different location but not when teleported to it's own location.