Caves in the Nether generates exposed lower bedrock

Summary:
The "old" small caves in the Nether expose the lower bedrock.
Examples:

Seed: 4815717814689529847
Coordinates: -18 9 259
Coordinates: 112 13 163

Steps to reproduce:

1. Create a world with the specified seed.
2. Visit the above coordinates.

Observed results:
The lower bedrock in the Nether is exposed.

Expected results:
Bedrock will be covered with lava or blocks.

Image: