REST API to get user's Atlassian account ID works intermittently

Issue Summary

When using the REST API to get user accountIds, it is not retrieving the AccountId value for the account where the username is used.

For example:

https://myinstance.atlassian.net/wiki/rest/api/user/bulk/migration?key=my_username
 https://myinstance.atlassian.net/wiki/rest/api/user/bulk/migration?username=my_username'

Steps to Reproduce

1. Use an API tool, a terminal or a browser tab so you can use the API.
2. Adjust it according to your current "username" or "key" parameters.
3. Run the API call.

Expected Results

The result should retrieve the user's Atlassian Account ID.

Actual Results

Only the username used in the call will be retrieved without the Atlassian Account ID information. For example:

{"results":[{"username":"my_username"}],"start":0,"limit":200,"size":1,"_links":{"base":"https://myinstance.atlassian.net/wiki","context":"/wiki","self":"https://myinstance.atlassian.net/wiki/rest/api/user/bulk/migration?username=my_username"}} 

Workaround

Export the users to retrieve the Account ID for each of the users within the instance.