[Trexmaster]

I've had this occur in my own house before. Any affected tiles had to be replaced by clicking and dragging from an unaffected tile and covering the void-space with whatever tiles.

The affected tiles remain after exiting and re-entering, and sometimes they revert back to being devoid of tiles, and sometimes they generate random dungeon tilesets within the affected tiles. (they remain fixed if you put new tiles over them, though.)

I decided to leave a small area of affected tiles in the floor of my house that was affected to see how it worked, since it seemed to happen at utter random (at first it was simply a void of tiles, black space that wasn't considered any house tile, then later it'd sometimes form dungeon tilesets).