Based on 1.11.2 decompiled using MCP 9.35 rc1; Code analysis potentially outdated

The bug

Mob spawners do not test if the entity to spawn collides after calling net.minecraft.entity.EntityLiving.onInitialSpawn(DifficultyInstance, IEntityLivingData) (respectively the methods overriding it). This could result in mobs suffocating if the method onInitialSpawn changed their position.

Note: For naturally spawned mobs this is tested.

How to reproduce

1. Encase a chicken and place directly above it a repeating command block

   teleport @e[type=zombie,distance=2..] ~ -255 ~
   

2. Create a spawner 4 blocks horizontally away

   /setblock ~ ~ ~ spawner{MaxSpawnDelay:5s,MinSpawnDelay:5s,SpawnCount:255s,RequiredPlayerRange:16s,SpawnRadius:1s,MaxNearbyEntities:255s,SpawnData:{id:"zombie"}}
   

   At some point a baby zombie starts riding the chicken even if there is not enough space above causing the zombie to suffocate