The bug

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 MC-477.

How to reproduce

1. Create a Superflat world with the following preset

   minecraft:stone,minecraft:air,2*minecraft:stone_slab
   

2. Search for Skeletons

   /execute as @e[type=skeleton] run data modify entity @s Glowing set value 1b
   

   The head of the Skeletons riding the spiders is inside the slabs

Affected classes

Based on 1.11.2 decompiled using MCP 9.35 rc1