Hey there,
Make sure you had enabled this plugin as shown in the screenshot.
https://take.ms/78Ynk
Go to
Backend > Components > Smart Search and try to search that user and see whether got index or not. If got, this means that the user has been indexed.
https://take.ms/GOJrb
If don't have, then press 'Index' button to index. After finishing index, try to search the user again and see.
https://take.ms/GOJrb
If the issue still persists, currently I have no ideas where goes wrong without access your site
because we need to troubleshoot on your site then only can know what issue causing this.
Perhaps you can consider to setup one of the staging site and provide us with your site backend and FTP access so we can have a better check?