The bug

The chunk NBT data includes a field called LastUpdate that stores the timestamp in ticks when the player last updated he chunk. It is updating erratically in spawn chunks. When a player is in range of a spawn chunk, the field sometimes updates and sometimes does not.

Attached is an image of a world where I flew about the map in creative mode, generating the terrain as I went. The LastUpdate for each chunk is plotted as a heat map, with blue being old, red being recent and pink and white being the chunks with the newest updates. The location of the spawn chunks are clearly visible as an area with spotty colouring in the centre on the right side.