What this bug basically allows for, is passive mobs to spawn on none grass blocks, such as stone, concrete, beacons, iron blocks etc etc. The only thing the game is really taking into account for where to spawn things, is biome. This also affects some hostile mobs too(like raids)
So for instance, in super flat savanna world, with only stone platforms, you can have llamas spawn. As another example, polar bears can spawn on stone in snowy biomes. As you can see from these couple examples, the block check mobs should be doing is clearly missing in recent updates.
Things like cats(from villages), llamas, horses, rabbits, polar bears, and turtles can spawn on any block, as long as its in the correct biome and correct light level. Llamas (only in savanna biomes?) and rabbits, can also spawn under ground. They should be surface spawns only. Cats also spawn inside blocks.
Most of this bug was fixed in 1.10, passive animals needed grass to spawn, drowns needed water, and so did guardians. Now in 1.11 passive animal spawning is broken again. But the drown/guardian fix persists.
This also affects raids. Raids don't do any form of block check, at all. If you can stand on it, they will spawn on. They will even spawn under ground, which is a massive issue.
Below you can see images of these bugs in action, most of which where taken in a super flat testing world, so condense spawns into one area. Oh, and the raids are not taking damage on magma, due to this related issue:
You can see a full demonstration and description in this video: https://youtu.be/ymgbqpCAOMQ