The Bug:

Ruined portals can break the bottom bedrock layer.

Ruined portals can generate at very low y levels when there are huge air exposed caves from around y -55 to y 64. This sometimes causes the blocks from ruined portals to override the bottom bedrock layer, thus allowing access to the void.

Here is an example:

Seed: 870041913950284822
Coordinates: /execute in minecraft:overworld run tp @s -0.47 -72.61 16.88 -138.90 -50.85

Steps to Reproduce:

1. Generate a world with the seed provided above and teleport to the given coordinates.
2. Look closely at the bottom bedrock layer.
3. Take note as to whether or not ruined portals can break the bottom bedrock layer.

Observed Behavior:

The bottom bedrock layer can be replaced with the ruined portal blocks.

Expected Behavior:

The bottom bedrock layer would not be replaced.