This is likely an issue with your style then, but it's hard to say without being able to see you. You should confirm whether it appears on a default style (and if not, disable all add-ons as well).
Thanks guys, I switched over to the default theme and usernames work fine there, so have isolated it to a theme issue. I'm guessing there was an automatic update which has caused the issue. Have a support ticket open with the theme guys and hopefully get it sorted.