AEM 6.4 - Rail Selector References

Hi all, I'm experiencing problems when using References from the rail selector.  When I navigate to a PDF and go to the rail selector drop down and choose "References", I'm expecting to see a list of pages that reference this PDF (image 1).  As you can see, I get zero results.  Image 2 is showing a page that is referencing the PDF via an Article Teaser component.  Does anybody have any pointers or suggestions as to why I experiencing this type of behavior in 6.4?  For what it's worth, we were previously running 6.2 without incident but since upgrading to 6.4 the problem has since presented itself.

Image 1: Shows the location of the PDF

Image 2: Known page that references the PDF