Just to be clear, this isn't a visual bug.
I knew this was discussed before so I did a search for "search username" on posts made by theymos with my
new tool #ad and I found the post I was looking for:
Each user has a display name, which is the only name ordinary users see, and a username, which is what you use to log-in with. Almost always, they are the same.
If I change a name for reasons of appearance, then I only change the display name. The username remains reserved, the user can still login using the username, PMs can still be sent to that username, "search member" allows searching for the old username, etc.
If I change a name for reasons of privacy, then I change both the username and the display name. In this case, it is possibly hazardous to reserve the old username, since it allows someone to test for the existence of that username, possibly defeating the privacy benefit. If you're worried about being impersonated, then simply don't seek to have your username changed.
In this case, the name was changed for reasons of appearance, so only the display name was changed. That's why the original name still shows up when you use the "find members" tool.
