Also affects current version of Confluence in all Cloud instances.

Steps to Reproduce in Self Hosted Confluence instances using LDAP:

1. Add a LDAP user as a watcher to a page
2. Remove the user from LDAP
3. Sync the LDAP directory in Confluence
4. View the page's Manage Watchers
5. Try to remove the Unknown User
6. Reload the page, and notice that the Unknown User still appears there:
   

Workaround

As a space administrator, remove the invalid user from the watchers list.

If the user disappears and reappears then do:

1. Find the user:

   select username from user_mapping where user_key in (select username from notifications where contentid = <YOURPAGEIDHERE>);
   

   PAGEID can be determined by clicking "Edit" on the page and grabbing the last number in the Browser URL.

2. Get your user_key by selecting the user from previous query and searching with:

   select user_key from user_mapping where username = '<YOURUSERNAMEHERE>';
   

3. Backup Database
4. Delete all notifications for that user:

   delete from notifications where username = '<YOURUSERKEYHERE>';
   

   This will also fix the problem for any other pages not that has that deleted user as a watcher

5. Flush Cache