Based on 1.11.2 decompiled using MCP 9.35 rc1

The bug

The method net.minecraft.world.WorldEntitySpawner.findChunksForSpawning(WorldServer, boolean, boolean, boolean) which spawns mobs in the world and the method net.minecraft.tileentity.MobSpawnerBaseLogic.updateSpawner() which spawns a mob for a mob spawner call the method net.minecraft.entity.EntityLiving.isNotColliding() before calling the method net.minecraft.entity.EntityLiving.onInitialSpawn(DifficultyInstance, IEntityLivingData), which can make a mob be a baby. This makes it impossible for baby mobs to spawn in smaller spaces.

Note: Fixing this might break existing mob farms.

How to reproduce

1. Create a Superflat world with the following preset

   minecraft:stone,minecraft:air,minecraft:glass;minecraft:plains;
   

2. Set the time to night

   /time set 14000
   

3. Use the /say command to list all zombies

   /say @e[type=zombie]
   

   → It won't find any zombies even though baby zombies would have enough space to spawn