Cannot @mention specific users in Confluence

Summary

When trying to @mention users in any Confluence page, it doesn't work. This only happens for specific users in the instance.

Environment

Confluence Cloud

Steps to reproduce

1. Create a new default page
2. @mention users

Expected behavior

Confluence should allow users to @mention any active user that has access to the application.

Workaround

Disable and enable Confluence application access for the user you cannot @mention.

OR use [~nameoftheuser instead of @.