In case a user is renamed in JIRA, she is shown as inactive and missing a display name in the REST API worklog. Please see following example. Attached is an image of how this worklog item is displayed in JIRA worklog tab.
The new name of this user is useris_two even though the REST API result shows the name as 'useris2' which is actually the user_key. I checked the JIRA worklog table and the AUTHOR is user_key value (useris2 in this case), so this seems to be only a REST API result displaying issue.
We are not sure since which version this issue has started as we discovered it in JIRA Cloud.