While walking to chunks that were not loaded at spawn, mobs often end up loading before the rest of the chunk, and when the rest of the chunk does load, the mobs don't move at all. Quitting and reloading the world solves the problem for the chunks immediately surrounding the player. This also occurs when mobs from properly loaded chunks move into chunks that contain non-moving mobs - a baby chicken walked into one such chunk and didn't move past the chunk border. When I reloaded the world, the baby chicken started moving again and had grown up, suggesting that these mobs still experience time while frozen, but it is not visible to the player.

I have dropped items in these improperly loaded chunks, but I haven't been able to pick them up until I quit and reload the world.

I have not found a specific pattern for the chunks that this occurs in, but a lot of the instances that I have found this happen in seem to occur along the X axis.

Steps to Reproduce:
1. load a world and walk several chunks away to load new chunks.
2. watch for chunks where the mobs load in a noticeable amount of time prior to the landscape.
3. look at the mobs around you. The mobs that were loaded in before the landscape do not move.