Flat World Chunk Loading Bug

I was playing Minecraft, on a single player flat world. I was proving to my friend that if you put a different block under a note block, it makes a different noise. I turned around, and I noticed that a couple of regular minecraft world chunks had spawned. There was also random spots in the piece of land where it went into a flat world. The coordinates of the land are X: 397, Y: 4 though 80, Z: 142. The World seed is 6835003468606631710.