User name shows taken but it is not

Summary:

We have a user who's username can not be updated

Content (required):

We have a user who's username can not be updated, "The username '20003542761' falls into the block of accounts you wanted to add, but it is already taken. Unable to update user info." It does not show in a search and it does not return on a orphaned user report.

Version (include the version you are using, if applicable):

Most recent

Code Snippet (add any code snippets that support your topic, if applicable):