The bug

Using custom world generation settings which set the minimum world height lower than the minimum height for the world generator, lava pools will occasionally spawn in the world's bedrock layer, allowing access to the void.

How to reproduce

1. Create a world using belowBedrockTest.json
2. Teleport to the location in question:

   /execute in minecraft:overworld run tp @s -7.58 -70.87 2276.52 47.70 -65.70