The bug

Spider and chicken jockeys only spawn the additional mob. This means spider jockeys will only spawn the skeleton and chicken jockeys only the chicken respectively set the IsChickenJockey tag of a nearby chicken to true without a zombie spawning on it.

How to reproduce

Create a Superflat world with the following preset to only allow spiders to spawn 3;minecraft:stone,minecraft:air,minecraft:glass;1; Set the time to night Use the following command to get to a nearby skeleton spawned as a spider jockey /tp @p @e[type=skeleton,c=1] → You will see that the skeleton is not riding a spider

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

It looks like the method net.minecraft.world.World.checkNoEntityCollision(AxisAlignedBB, Entity) is not working correctly and counts entities riding the same entity as colliding while it should ignore them. See also net.minecraft.world.World.getCollisionBoxes(Entity, AxisAlignedBB) .