Bedrock still can generate below the old chunks after upgrading the world to 1.18 generation

I was exploring my 1.17 world on 1.18 beta and i noticed that some part of the chunks can generates bedrock from y 0 to y -64 on my 1.17 survival world. The other chunks have been upgrade properly, but only some of the old chunks can generates bedrock from y 0 to y -64.