Issue Summary

This is reproducible on Data Center: Yes

Steps to Reproduce

1. Created a dummy user - testuser in my Jira instance. 
2. Created a page in confluence, linked that page in Jira, and mentioned 'testuser' in the confluence page.
3. Anonymized the user in Jira 
4. Sync'd the Jira directory in confluence 
5. we could see "UNKNOWN USER(tetsuser)" getting displayed 

Expected Results

Confluence should display the anonymized user correctly just like it's displayed in Jira - please check the assignee field - testuser changed to user-0da92 once anonymized.

Actual Results

we could see "UNKNOWN USER(tetsuser)" getting displayed 

Workaround

In the case that the user has already been "unsynced" by anonymising the user in Jira, it is possible to anonymise the user in Confluence with the following steps

1. Add a user to the local Confluence user directory with the same username as the "unsynced" user
2. Delete the newly created user per our standard anonymisation process

The bug is similar to - https://jira.atlassian.com/browse/CONFSERVER-39209, and as such that bug may be worth reviewing.