Normally passive mobs are only able to spawn on grass blocks, this bug allows them to spawn on almost anything. Mobs that i know are affected include: Llamas, horses, turtles, polar bears, and rabbits. Below you can find the specifics for each mob, and how to reproduce.

Llamas:
In a savanna biome, its easy to find packs of llamas spawning on most blocks, above ground. blocks tested include: Stone, cobble, smooth stone, quartz, wood, iron blocks, clay, beacons.

Horses:
Also in a savanna biome, you can find horses spawning on the same things llamas spawn on(as far as i have tested)

Turtles:
in beach/ocean biomes, turtles have been seen spawning on block platforms above water. They can spawn on the same things as llamas (as far as i have tested)

Polar bears:
In snowy tundra biomes, polar bears can spawn on most blocks. Some think that snow layers affect the things they spawn on, it does not. They can spawn on the same things regardless of it having snow layers, or not. They spawn on the same things as llamas (as far as i have tested)

Rabbits:
Pretty much the exact same as polar bears, nothing special here.

General guide on how to reproduce:
1. Load up a normal world in creative, in peaceful mode, mob spawning off, and daylight always on.
2. Find the biome you need for the mob you want to test.
3. Fly up to y150, use /fill ~30 150 ~30 ~-30 150 ~-30 stone
4. Fly up to y180, turn mob spawning on. Wait, a longgg time. Passive animal spawns are very slow.
5. Use /kill @e often to clear out the cap, and use the fill command to switch out the blocks for what you want to test.
6. You will see the mobs listed, spawn on the blocks listed.

expected results
Animals like llamas and horses, should only spawn on grass.
Turtles should be limited to sand, maybe grass, and what ever else they normally spawn on.
Polar bears/rabbits should be limited to grass, snow blocks, and ice variants. Along with whatever else they normally would spawn on.

You can see some demonstrations of these bugs here: https://youtu.be/ymgbqpCAOMQ

Also, yes, this is technically a duplicate of MCPE-39727 i was told by auldrick and mega_spud to make a new report for this issue.