Hi Malcolm -- sorry you are having an issue with your story. To my knowledge, we haven't received any reports of disappearing tabs. There is a capability to hide tabs (in the Organize panel), but I just looked at your story data and that is not the case here.
The only other thing I can think of is custom HTML. Have you added any custom HTML to this story in the source view of the side panel editor? I suppose it's possible that some HTML styling may have affected some of your tabs? If that's not the case I'd suggest that you contact Esri Support and they can help you troubleshoot further.
Owen Evans
Lead Product Engineer | StoryMaps