Chunks reset to 1.7 generation in older world

When walking through my world I've had since 1.2, a chunk near my base had ben reset to 1.7 generation. This freaked me out a bit, so I went looking but couldn't find any other chunk that were reset fortunately. Still afraid this might happen though.

I've included a screenshot. If you need me to test anything or provide extra information, let me know.