My Hardcore world in the new snapshot generated bedrock around y=0 without ever having opened on a version of the game without the extended height. I have gone below y=0 before in this world but now there is a floor of bedrock where normally my minecart rail would be. I have only launched snapshots of the game since the inception of this world, so I don't know why it would have generated. I have tried going back to snapshots as early as 21w17a so see if this was something with the new snapshot last week but the bedrock did not disappear. It wasn't there on the previous times I've played in the last week. Sorry for the word dump, just trying to provide as much information as possible. Would really like to go looking for some geodes :').