There has been no change to the site; this would be an issue with the font library installed on your computer. Without knowing what recent changes to your computer have been made, I'm not sure how to advise you to fix it.
Usernames will be displayed in Helvetica Neue, Helvetica, Arial, or the default sans-serif system font configured in your browser. At least one of those fonts is installed by default on all major operating systems, so if you're seeing anything else, their font file has either gone missing or has become corrupt on your machine.