Why can't I rearrange the size variant XS in front of S?

[Kpdesigncenter]

Hi - I added an XS size variant but when rearranging the XS in front of the S and clicking on save I keep getting "Unsaved Changes". Is this a bug? 

I've tried to duplicate the product, removed the XS items and re-link them but still no luck. Any suggestions will help.