When creating a Test Execution from a requirement the assignee user picker throws an error

Description
When creating a Test Execution or a sub test execution from a requirement the assignee user picker throws an error

How to reproduce

1. Create a requirement(Story or epic) and add 2 Tests;
2. Go to User Management and change the username admin to admin2;
3. Create a Test execution or a Sub test execution ;
4. The error appears.

Actual result/Attachments

**An error appears: User 'JIRAUSER10000' does not exist.

Expected result

The Test Execution should be created correctly.

Version

• Validated in 5.1.0, but should also happen on earlier versions.