I already have a solution for this but am writing it up in case others run into the same issue.
After creating a new user, my email address was showing up as the username of the new user. (This site uses email addresses as the username;
useEmailAsUsername is set to true in config/general.php.) Here's how it looked in the user list:
It also showed up on the user page:
I thought I might have stumbled on a bug related to
useEmailAsUsername. Silly me. Craft support (hi, Brad) wasn't able to reproduce it and suggested it might be an auto-fill browser extension misbehaving. Sure enough…