I got on my Legacy world that I completely deleted down to the last layer of bedrock to terraform for the first time since the new 1.17 beta, I noticed a row of chunks got added and thought nothing of it, and started to delete them via commands, I went to look around the world some more and noticed that not only had a row of chunks been revealed, but a row of chunks that I slowly had been terraforming for months was hidden behind the new world border.
I don’t know if this is a bug or not, but I hope you can fix it, because that row of chunks was essential to my build.