Three em-dash problem in bibliography

In bibliographies it is customary, in some styles, like the Chicago one that I am using, to replace the name of an author, where is appears more than once, with a three-em dash. This should display as one continuous line that is three-ems in length. In Pressbooks, the epub version of the output does this but the pdf and webbook display as literally three dashes.

Here is an example of what the pdf rendering of the Graham theme bibliography looks like. Note, three separate dashes not one long dash, as it should be.