hoverlabel.grouptitlefont getting reset

If I set `hoverlabel.grouptitlefont`, it appears to initially work - if you watch carefully when you first hover on the graph you'll see the font you set (in the codepen, a very large font) but then it quickly gets replaced by the default font it inherits from `hoverlabel.font`
https://codepen.io/alexcjohnson/pen/QWzrzWw?editors=0010