Cannot mention specific user in pages or comments at all

When trying to mention a user ( with @mention or [~mention] ) the user does not show up as an option. The specific user can't be mentioned by anyone on pages or comments. 

Steps to Reproduce

1. Create a page or comment
2. Type "@user"

Expected Results

• You can select the desired user to mention

Actual Results

• The user doesn't show up as an option and cannot be mentioned

Notes

• This will also make it impossible to add the user individually on space permissions and page restrictions.
• This is a very specific bug, but the symptoms are common. If you can [~ mention, or if you are unable to mention more than a few specific users, this bug is not what you are dealing with. 

Workarounds

• First one:
  • Contact our support team through https://support.atlassian.com/ and include the following information:
    • The user(s) being affected
    • A link to this report (or simply mention CONFCLOUD-67259)
• Second one: 
  • Go to the site's administration > Users > Select the user > change the User Role to Trusted, refresh the page and then, change it back to the previous role (Site administrator/Basic).