Zombies of village siege spawn despite gamerule doMobSpawning being false

The bug

Zombies of a village siege spawn even if the gamerule doMobSpawning is turned off.

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. Turn the gamerule doMobSpawning off

   /gamerule doMobSpawning false
   

3. Get to a nearby village, for example by using /locate Village, and wait