Mud does not generate underwater in mangrove swamps

The bug

Mangrove swamp biomes generate mud as their surface block. However, it only appears to generate in areas above water - underwater regions appear to use dirt instead.

How to reproduce

1. Find a mangrove swamp biome
2. Pay attention to underwater areas

Expected results

Mud would generate as a surface block above and below water.

Actual results

Mud only appears above water.