Changing the Component Lead of an existing Component throws 'the user <username> does not exist' on save due to mismatch in user key and username

Summary

Cannot set usernames with upper case letters as Component Leads for existing components.

Environment

• Use of upper- or mixed case for the logon name (username);
  
  

Steps to Reproduce using Active Directory

1. Create a username with upper- or mixed case in AD. For example: ROCKY;
2. Sync the directory to JIRA;
   
3. Create a component in any project - under Project Settings > Components and any user as the Lead;
4. Once the component is created, click Edit under the 'Actions' menu;
5. Search for the user created in AD to be the Component Lead;
6. Save;

Expected Results

User should be assigned as Component Lead.

Actual Results

When saving the user, it will throw an error:

The user <username> does not exist.

Notes

• This error only affects editing Component Leads. Creating a new component using a mixed case Component Lead will work as expected.
• The problem affects Internal Directory users with mixed cases as well.

Workaround

Use the JIRA REST API to edit the component and update the Component Lead:

• https://docs.atlassian.com/jira/REST/7.2.2/#api/2/component-updateComponent
  We've also put together the following KB article that shows how to use PostMan to modify the Component Lead via the rest api:
  https://confluence.atlassian.com/display/JIRAKB/How+Update+a+Component+Lead+with+the+Rest+API