When in a singleplayer or multiplayer world, the spawn chunks in the overworld stop being loaded after a minute or so. This has always been the case. It used to be possible to force the chunks to stay loaded by dropping an item in an ender portal.

Since the latest snapshot, this is no longer possible. I'm not sure if it's a bug or a bugfix, but I do know that it's extremely useful to have some way of keeping chunks loaded, for example when using command blocks.