Zombies casually spawning inside village

The Bug

Same as title: Zombies casually spawning inside village

• Underground breeding space (all entrances sealed while AFK)
• Light level always over 8
• "Village" only has 8 doors and 1 villager, and it's 8 blocks above the breeding space
• Happens in both daytime and nighttime.
• ...and boom, zombies, and all the villagers that you breed turned into zombie villagers
• Sometimes zombies even spawn within 10 blocks away from you

Sieges do not work this way, right?