All leaves in the game, with the exception of mangrove ones, use separate textures for its default look and its opaque look when fancy leaves are turned off. Changing terrain_textures.json to make mangrove leaves use multiple textures in the same way as the other ones doesn't work either, so resource pack creators are even missing the ability to change it if desired.

Steps to Reproduce:

1. Import the attached resource pack for easier viewing:
   -> All opaque leaves textures were modified to add an easy visual identifier. terrain_texture.json has also been properly edited to supposedly make use of the separate opaque mangrove texture.
2. Turn off fancy graphics (or fancy leaves, if available);
3. Place every type of leave and compare.

Observed Results:
Mangrove leaves don't use separate opaque textures, and are unable to.

Expected Results:
Mangrove leaves work the same as all other leaves.

Screenshots/Videos attached: Yes

Notes: MCPE-120202 allows for both the regular and opaque textures to be seen at the same time, that's what was used to take the screenshots.