Since MC-279939 was apparently resolved as WAI, the parity issue was redirected here.

Steps to Reproduce:

1. Generate a new creative world (preferably using the seed 4000051, as it has many vast mangrove swamps next to spawn, but you can test it on any seed as many times as you'd like). Make sure to enable cheats when making the world.
2. Place 4 command blocks on the ground in your spawn chunk, and set them all to "Repeating" and "Always Active".
3. To help observe the ground clearly, put each of these commands into the four command blocks, respectively:

   /execute at @p run kill @e[type=minecraft:item]
   /execute at @p run fill ~-20 ~-10 ~-20 ~20 ~8 ~20 air replace minecraft:mangrove_log
   /execute at @p run fill ~-20 ~-10 ~-20 ~20 ~8 ~20 air replace minecraft:mangrove_roots
   /execute at @p run fill ~-20 ~-10 ~-20 ~20 ~8 ~20 air replace minecraft:mangrove_leaves
   

4. Use the following command to locate a Mangrove Swamp, and teleport to it:

   /locate biome minecraft:mangrove_swamp
   

5. Fly around and look for Firefly Bushes.

Expected Result:

You wouldn't find any firefly bushes in the biome as per the resolution of MC-279939.

Observed Result:

You do.