I was doing a storyboard for a beatmap and I was used to saving by clicking "esc" and then there being a warning asking "Do you really with to save changes to this beatmap?" It only appears after editing hitobjects and/or -sounds, but not after editing the storyboard. I just did a storyboard for about 20mins and because of this I lost all my progress. So my suggestion would be to add this feature when making unsaved changes to the storyboard too.