Sand into Grass Bug

I opened up my minecraft world. Which was created in version 1.6.4. Although I have previously played in it with version 41b (the latest snapshot as of 14/10/2013). I was building in one area. Which was in a desert biome. Then, I left the area and flew to a different area so the chunk I was in was unloaded. Upon reloading the chunk an area which was all sand and desert had now turned into grass. And, within moments it started to rain in that area. Whilst the rest of the desert was still without rain, which is how deserts should behave. I flew over the now grassy area and my f3 menu told me it was now a plains biome. Bearing in mind, this wasn't a newly generated chunk, but a chunk which had been generated, built on, then unloaded and loaded back up. I have searched the bug tracker but found no other reports of this occurring. So, I'm sorry if someone else said this before me. I just thought I'd report it just in case no one else has. Enclosed is my first look at the chunk and also me with my f3 menu open. Also, in the chunk I had placed one grass block near the river. So, I don't know if that has something to do with it.