Villager Despawn

Hello Mojang team I play Minecraft on my phone. I lived in a village there and saved villagers there. Now I've built a house especially for the villagers and every time they turn into zombies, how can that be when I close the doors with wood and there's nowhere else a gap where a baby zombie can get through This is a bug and should be fixed immediately because it spoils my desire to play minecraft and I actually love Minecraft best regards HaYaMaIn (Handy Edition) BiggestNoobEU (PC)