Here you go
Another screenshot, of a different book (and the book itself). Left shows the book before Hyphenate This! applied, right shows it with. AFAIK, the plugin hasn't changed for ages apart from the conversion to QT5, and I have not seen anything weird after that, using Calibre 1.48.
PS: I remember that Sigil had this exact same issue, so it may have something to do with the settings or configuration of the browser component.