The bug

The way in which bedrock generates at the bottom of the world allows for certain arrangements that can permanently trap the player to be created. If a player end up getting trapped inside one of these regions, this can effectively result in a singleplayer world being softlocked if the player has no ender pearls, and cannot die and respawn to escape the situation (e.g. for Hardcore worlds, or Peaceful worlds if the player has no harmful items).

How to reproduce

These instructions use Creative mode for quickness, but the region in question can still be located in Survival and similar instructions performed.

1. Create a Creative world with the seed 1
2. /tp -21.70 -62.00 -18.54
3. /summon tnt (do this a few times to clear out breakable blocks, and also have a bucket ready to remove lava)
4. Get an ender pearl
5. Switch to Survival mode
6. Throw the ender pearl into the two-block-high hollow region shown in the screenshots attached

Once the player is inside of this region, the game is effectively softlocked, as (ignoring switching to Creative) there is nothing that can be done to escape this situation without either dying, receiving help in multiplayer, already having another ender pearl or using external editors.

How to fix

Should be relatively simple. Instead of generating bedrock randomly per layer, it could instead be made to generate in a similar fashion to how sandstone transitions to sand in deserts: any bedrock block will always generate bedrock below it, making the bedrock layer more like a 2D heightmap than layers of 3D noise. Structures like these would therefore be incapable of generating in new chunks.