Changing administrator in board only recognizes old username

Summary

When editing the administrator in a board in the view with 'groups' and 'users', the only username recognized is the user's first username, in case it has been changed.

This is reproducible in JIRA Cloud.

Steps to Reproduce

1. In a new instance with default permissions, login with a user belonging to these groups only: administators, site-admins (and if it is sysadmin, you can also belong to system-administrators)
   • This is because the fields for configuring of administrators change depending on permissions
2. Create a Scrum board
3. Create another user with username 'user1'
4. Change the user's username to 'user2'
5. Go to the board's 'General Configuration' page
6. Click 'Administrators' to change the administrators
   • Two fields must appear: 'Groups' and 'Users'. Otherwise, this bug won't happen
7. Add 'user2'

Expected Results

The user2 (previous username user1) is added as administrator.

Actual Results

The user is not added. This error appears:

It reads:

Notes

The field expected 'user1', though it's actually 'user2' its current username. This is actually stored and kept by JIRA.

Depending on the configuration, when editing this field, this will be visible:

Instead of 'Groups' and 'Users', only one field is visible (that’s why the groups were limited to reproduce the problem). In this case, JIRA will accept the correct usernames.