All the old World I tried to open with experimental features toggle on the generate with bedrock (block) in the height level from 0 to -64. New world generated don't face this problem but any world from previous version does have the issue.
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duplicates 1
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New cave generation won't be on old chunks, to prevent affecting things you may have built in those. In order to access that cool new generation you need to be on new chunks (those who don't have the bedrock between 0 an -64). Just go to places in your world you've never been before, and then you'll find the caves : D
Thats suppose to happen because when you enable the toggle, new chunks generate down to -64. If there was a new chunk next to an old one, and you mine towards the old chunk at any height level less than 0, you will have access to the void. This also makes the world uneven. To fix this, the bedrock on old chunks will be extended to -64 to prevent players from breaking out, and to even out the bottom of the world.