The bug

Zombies spawned by a village siege do not spawn centered on a block, instead they use the block position coordinates. This can, combined with MC-59440, result in suffocating zombies.

How to reproduce

Village sieges only happen

• when the moon is at the zenith
• when a player (not being a spectator) is nearby
• when no second village is close
• when the village has at least 10 doors
• when the village has at least 20 villagers
• with a chance of 1 / 10

Because a siege only happens with a chance of 1 / 10 it is easier to cause one by having a clock (for example repeater) setting the time to day, waiting a short moment (for example one redstone tick), setting the time to 18000 and then waiting a little bit longer.

1. Create a Superflat world with the preset "Classic Flat"
2. Get to a nearby village, for example by using /locate Village
3. Setup a repeating command block with the following command

   /entitydata @e[type=zombie] {NoAI:1b}
   

4. Look at the zombies spawned by a village siege
   → They will be in the middle of four blocks

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.village.VillageSiege.spawnZombie() does not add 0.5 to the X and Z coordinate when calling net.minecraft.entity.Entity.setLocationAndAngles(double, double, double, float, float).