Summary:
Some leaves on mangrove trees are generated completely separate from other leaves (some only touch corners). I've looked at the Mangrove Swamp Biome in java and it looks like all the leaves are touching each other's sides.

Seed: 7159122465189192029
Coords: 4424 85 164
Coords: 4239 95 29
Coords: 4463 78 485 

Steps to reproduce 1:

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

Steps to reproduce 2:

1. Create a flat world.
2. Write the command /fill ~1 ~ ~60 ~1 ~ ~ mangrove_propagule
3. Set the randomtickspeed game rule to a high value.
4. Look at the tops of the trees for flying leaves.
5. If no problem is found, repeat step 2.

Observed results:
Some leaves are generated completely separate from other leaves.

Expected results:
All leaves will be in contact with other leaves sides.

Image: