Boxes around capital letters and numbers in Adwaita Sans

I recently came across an issue on a website that enables the ‘ss06’ opentype font feature. For some reason, when this feature is enabled, boxes appear around capital letters, numbers and other special characters:

the html for the test document:

<!doctype html>
<html>
  <head>
    <meta charset="utf-8" />
    <style>
      .a {
        font-family: "Adwaita Sans";
        font-feature-settings: "ss06" 1;
      }
      .b {
        font-family: "Adwaita Sans";
      }
    </style>
  </head>
  <body>
    <h1 class="a">This Is a Test! 0123456789</h1>
    <h1 class="b">This Is a Test! 0123456789</h1>
  </body>
</html>

This behaviour appears in multiple browsers (firefox, chromium, gnome web)

I’ve tried messing with fontconfig, but to no avail. Can someone provide general pointers into where I should be looking to determine whether this is a fontconfig issue or a bug in the Adwaita Sans font? Thanks!

I am running F42 (workstation edition) and made sure to update all packages to the latest version. I’m guessing this has something to do with fontconfig or the Adwaita font itself. Since the font is relatively new , as far as I understand, it might be a bug in the font itself.