Zombies spawning as reinforcement are not centered on a block, instead they use are spawned between 4 blocks.
- Set the difficulty to "Hard"
- Set the gamerule doMobSpawning to true if it is not already
- Set the time to night or make sure there is no light in the area around you
- Setup a repeating command block with the following command
- Summon a zombie using the following command
- Hit the zombie a few times
- Search for the spawned reinforcement zombies, if there are no other zombies you can use the following command
→ You should see that the zombie is not centered on a block
Based on 1.11.2 decompiled using MCP 9.35 rc1
The method net.minecraft.entity.monster.EntityZombie.attackEntityFrom(DamageSource, float) does not add 0.5 to the X and Z coordinate it uses for spawning the zombies.