This is maybe a bug, maybe behavior by design, so I’ll use the bug category and we can go from there
I ran into an issue signing up for an account at the Boingboing BBS. Specifically, to claim my meta.discourse.org username, I had to sign up using the e-mail address I originally used to register here at meta, rather than the e-mail address I’m currently using. I switched addresses here at meta for gravatar reasons; it looks, though, like the reserved username database retains the registration e-mail rather than the current one.
This might lead to a situation where users are unable to claim reserved usernames—if, for example, a user no longer has access to the account that they originally used to register.