It's not a bug. It overwrites the file by design. The old model was revert-by-default. You had to manually save your changes. The new model is save-by-default

It's not a bug. It overwrites the file by design. The old model was revert-by-default. You had to manually save your changes. The new model is save-by-default. You have to manually revert your changes. In the "File" -> "Revert To..." menu you have multiple options, such as revert to last saved document and revert to last opened document. These persist even after restarting the application. I think the new model is far better design.