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.

{
"self": "http://jira63.dev:8631/rest/api/2/issue/10303/worklog/13101",
"author": {
"name": "useris2",
"active": false
},
"updateAuthor": {
"name": "useris2",
"active": false
},
"comment": "",
"created": "2015-06-10T12:39:42.000+0200",
"updated": "2015-06-10T12:39:42.000+0200",
"started": "2015-06-10T12:39:00.000+0200",
"timeSpent": "1h",
"timeSpentSeconds": 3600,
"id": "13101"
},

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.