That is the way to do it. Can you take a screenshot of what you are seeing when you expect the short title to be shown, upload it to an image hosting site (Imgur, Dropbox, etc.), and post a link here?
It should definitely be a statute -- my recollection was that OSCOLA doesn't want supra numbers for cases and legislations. Are you sure that's required?
If I make the item a bill, it shows the previous footnote number, but won't display the short title.