Mod Version: 1.3.5.1
Happens in Mystical Village 2 v3.5 modpack singleplayer, may be mod conflict

Over time they pile up in the middle of the spawner, clipping inside of each other. WAILA type mods show a zombie standing there. They can be damaged once before they turn invincible and can't be hit by anything. Can't be killed with lava. Switching game difficulty to peaceful does nothing, they continue to stand in the same spot impervious to everything. Restarting game clears them, but they come back quickly. It wouldn't be a significant problem, but it does cause problems with Ancient Warfare NPCs who seek out and attack them, despite not being able to kill them, soft locking the NPC AI (probably has similar issues with other mods).