After removing an user from a site, the user still can be mentioned in an issue by using @ in description and comments fields

Issue Summary

If a user was mentioned at least once in an issue and later this user is removed/deactivated from the organization/site, the user will still show up as a pickable user in the user picker dialog when the "@" symbol is used.

Steps to Reproduce

1. Mention a user either in the description field or in a comment of an issue
2. Deactivate/Remove the user from the organization/site
3. In the same issue where the user was previously mentioned, try to mention someone else. The removed/deactivated user will show up and can be picked.

Expected Results

• While the user has access to the organization/site/project and with the right permissions the user can be mentioned within an issue.
• If the user is removed/deactivated from the organization/site the user should not show up as a pickable user in the dialog.

Actual Results

• While the user has access to the organization/site/project and with the right permissions the user can be mentioned within an issue.
• If the user is removed/deactivated from the organization/site the user shows up as a pickable user in the dialog.
• No notifications are sent to the user after it is mentioned.
• A message is displayed if the tag is clicked: 

  Oops, looks like we’re having issues
  Try again and we’ll give it another shot

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available