I can't replicate this issue with my Lightroom 5.5 on Windows 7
However, if you don't mind me thinking out of the box, it seems like you are doing a lot of work to obtain a result that can be done much simpler, if you are willing to use a collection for this purpose. Simply drag the edited photo into a "Best Of" collection in Lightroom (no exporting needed), and you shouldn't have any trouble getting the proper sort order inside the collection. That's a lot less work than exporting! Saves disk space too! (And bvefore you object and say you have reasons why you need these in a folder for non-Lightroom purposes, that's easily solved by using a collection ... just do the export from the collection into a "Best Of" folder)