It isn't a bug, it works as expected: it toggles the real time overlay in your current session. It does not change your user configurations. To do that, locate your local `user.cfg` file (see https://trac.wildfiregames.com/wiki/GameDataPaths ), and delete the following line:
overlay.realtime = "true"
Alternatively, launch the game, click Settings (or Menu) → Options → General, untick Realtime Overlay, click Save, then Close.