Renamed users are unable to add Request Participants via email

Summary

Service Desk Customers (both portal only and AA) whose user_name is different from the user_key on the app_user table, are unable to add Request Participants via email (Cc).

Environment required

• A Service Desk project with an email channel configured.
• A user that had their username changed. (It used to be possible via the instance's admin panel)

Although usernames can no longer be changed, they still exist in the backend and are referenced by some functionalities of the application.

Steps to Reproduce

1. Create an Atlassian Account customer.
2. Change the account's username (contact the Atlassian Support to have that done)
3. Have the user send an email to the Jira Service Desk handler, Cc'ing any other user.

Expected Results

• The issue is successfully created.
• The CC is added as a Request Participant of the ticket.
• No errors are displayed in the log.

Actual Results

• The following error is displayed in the Handler's processing log:

  Mail handler throws exception: Unexpected error while running action as user 'xxxx' 

• Despite the error, the ticket is successfully created.
• No Request Participant is added to the ticket.

Notes

This also affects comments done via email.

Workaround

• Log in the portal and add the other customer as Request Participant.