Minecraft – Why are NPC houses halfway built

Maps show the blocks that would be visible from a bird's-eye view. For example, if you made a large canal, it would be visible on the map as blue. The maps are dynamic, so any changes to the world are shown. However, only the top-most block at that particular X/Z coordinate would show – because of this, nothing underground would be visible (since the map would only show the ground above).

These are called witch huts, here is the information from the wikia:

Witch Huts generate in swamp biomes or, very, very rarely, other biomes. Witches can spawn inside of them. The huts consist of spruce Wood Planks and oak wood. Spruce wood stairs make the roof borders, 2 fences decorate the entrance and one fence make the entrance window bars. Inside is a crafting table and an empty cauldron; on the windowsills are potted mushrooms. There's no valid way a witch can get back up into the hut if it falls off it.

There are no chests or any other loot inside. It is possible, but rare, for a mushroom to generate inside the Hut while not potted, with the potted one still in place. Strangely, they are just high enough for the witches to fit in, but their hats are in the ceiling, making the animation look broken. They seem to be based on the design of Baba Yaga's hut, a witch from Slavic folklore who lived in a hut that walked around on giant chicken legs.

Source: Witch Huts