The method net.minecraft.entity.EntityLiving.onInitialSpawn(DifficultyInstance, IEntityLivingData) (or rather the methods overriding it) does not check if the additional mob for spider or chicken jockeys collides when they are spawned.
This report is not about ridden entities not considering the passenger height when moving, suffocating the passengers. That is reported as
- Create a Superflat world with the following preset
- Search for Skeletons
The head of the Skeletons riding the spiders is inside the slabs
Based on 1.11.2 decompiled using MCP 9.35 rc1