Mangrove trees will fail to generate if their roots do not hit a solid, unreplacable block before reaching their maximum root length; attempting to grow a mangrove tree over void/at the very edge of a floating island (for example, using the end dimension), or in a world without a bedrock layer and all made of mud/air, will cause generation to fail.

Removing mud from the "MANGROVE_ROOTS_CAN_GROW_THROUGH" tag nullifies case 2, and mangrove trees generate properly in that instance.

How to replicate:

Case 1, through air:

 - Place a single dirt/mud block suspended above a void (end for ease) or at the edge of a floating island (end for ease)

 - Place Mangrove propagule, apply bonemeal

 - Tree fails to generate (fig. 1)

Alternatively, using /place feature minecraft:mangrove will provide feedback in the chat window (fig. 2)

case 2, through #mangrove_roots_can_grow_through:

 - Create a superflat world with any number of mud blocks

 - place mangrove propagule, apply bonemeal

 - tree fails to generate (fig. 3)

Alternatively, using /place feature minecraft:mangrove will provide feedback in the chat window (fig. 4)

Expected behaviour

Mangrove trees generate properly above voids, with hanging roots.