I have same very old (alpha) chunks in my world.

When I update to 1.18 pre-release1 then in these chunks the bedrock layer is replaced by deepslate and the area below y=0 is filled with new stuff. So far as expected.

BUT: The lowest layer of bedrock in these alpha-chunks is not completely solid, it has "holes" (that are usually filled with stone, or lava), sometimes these "holes" go through the whole bedrock layer. See attached image "1.17_2021-11-11_22.50.48.png" where I dug out a large area down to bedrock.

After updating to 1.18 pre-release1 everywhere where the bottom block of the bedrock layer is not bedrock (e.g. stone, lava, air, whatever...) there is now a hole in the newly generated y<0 area. These holes go down to and through the new bedrock layer at y=-64. See attached images "1.18_*.png" (newly generated bedrock layer at y=-64 viewed from below under old alpha chunks)

Steps to reproduce:

Create a world in Minecraft alpha (e.g. 1.1.0 or earlier), update to beta 1.8.1, then to 1.6.4, then update to 1.18 pre-release1

Expected behaviour:

The newly generated area below y=0 is normal.

Observed behaviour:

The newly generated area below y=0 has lots of vertical holes that go straight throught the bedrock layer and into the void. See attached images "test_*.png" (spectator view from ca. y=64 down)

Scope:

This will affect all players that have started their world in alpha1.1.0 or earlier and kept playing that same world. This effect makes the newly generated areas below y=0 basically unusable.

Proposed solution:

The algorithm that decides whether to put or not put new stuff below y=0 should ignore 1 and 2 block "holes" in the old bedrock layer. Larger "holes" might exist, but are probably seldom enough that they are more a curiousity than a game breaker.