San Francisco on Thursday filed a lawsuit against the developers of a sinking and tilting luxury high-rise, claiming they knew about the problems but did not disclose the information to potential home buyers as required by law. Millennium Tower was completed about eight years ago and so far has sunk 16 inches into the soft soil and landfill of the city’s crowded Financial District. The sinking has been uneven, creating a 2-inch tilt at the base and a roughly 6-inch lean at the top. City Attorney Dennis Herrera filed the suit against Mission Street Developers LLC as a cross-complaint of a previous lawsuit filed by homeowners against the Transbay Joint Powers Authority.