Mud in mangrove swamps is only one block deep

The bug

Mud which generates in mangrove swamp biomes is only a single layer thick. This is inconsistent with other biomes such as deserts and badlands, in which the surface block is several blocks thick, makes vertical inclines look ugly due to the exposed dirt standing out, and overall seems very wrong.

How to reproduce

1. Locate a mangrove swamp
2. Break a mud block
3. Look below that mud block

Expected results

There would be more mud.

Actual results

There is dirt.