In bedrock 1.21.40.22 mojang updated the rules under which Bats spawn in the world:

1. Bats can now spawn at any height, provided the area is covered and sufficiently dark
2. Bats may now happen to spawn above the surface

However, it doesn't work well.

1. They use the ambient population pool, which only has 2 max mobs underground density in 9*9 chunks and 0 mob density on the surface.
2. The density limit on the surface is 5 which higher than ambient pool.
3. The herd size of animals is 8 which higher than ambient pool and density limit.
4. There's no spawns_on_surface condition 

Expected results:

1. The bat should happen to spawn above the surface because they add density limit for surface.
2. Every time bat should spawn 8 instead of 2,
3. The total amount of bat in loaded chunks should be parity as Java edition.