World deletion fails

• Create a new non-flat world, give it a sample name like "testworld"
• leave to menu, delete this world
• looking to the saves folder shows you, the world is not deleted
• creating a new world, give it exactly the same name as the first one, but change it to be a superflat one
• loading this world results in either loading the old one, or loading mixed chunks from the non-flat and the flat one, like on the screenshot