Mobs have the ability to spawn inside Village Houses as there isn't enough Light in the houses to prevent spawns. This makes villages too vulnerable.
The picture shows the Light Level from the position in the house.
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I suppose the "bug" is that the villager AI guides villagers into the generated houses of a village in a purported attempt at staying safe from zombies, which makes no sense if the zombies can simply appear inside the houses. Really, it's rather silly that the pre-made village houses aren't lit well enough to thwart mob spawning.
Hostile mobs spawn at a light level of 7 or less. I fail to see a bug here.