One user can't be tagged in description for issue creation, but can after issue is created

[James Wall]

Hello, all I could find when searching is to make sure the browse user rights were there, and they are. Other users show up to tag, but one doesn't. But after issue is created, user can be tagged in description or comment. And on creation the user can be found to tag on ID, but not name. It's obviously an anomaly, but user is adamant about a resolution. Any ideas? Thanks, Jim