How to create a video for a map?

Videos usually don't disappear; most likely, you're experiencing a display bug, and this happens frequently. The video doesn't actually vanish; it's still there, functioning correctly, and can be found within the map archive for editing. However, it's unfortunately no longer possible to edit it directly within the editor. Although, I don't quite understand how you were able to "open only the video" without the map... The video is stored within the map archive. If you performed any actions with the editor without opening the archive, it's understandable that the progress wasn't saved.