Cannot change assignee/reporter if the user is an 'unknown user' from a server import

Issue Summary

When migrating from Jira Server, Jira still preserves the users that remained on other user directories that are synced with Jira (such as AD or ay other), if any of those users, be assigned or the reporter of a ticket, it may break it and you won't be able to perform changes in the ticket, including changing the assignee to someone else.

• This scenario can only happen if you come from a Jira Server Version
• Whenever an issue is assigned to an 'unknown user' it'll look like this:

Steps to Reproduce

1. Identify an issue assigned to an unknown user (example below)

2. Attempt to change the assignee or reporter of the ticket

Expected Results

Jira will still allow you to operate changes on the issue, as this "unknown user" scenario is expected as Jira doesn't migrate users from different directories automatically (see JRACLOUD-71339)

Actual Results

The issue is broken and can no longer be changed or have those fields fixed

• The following call will fail and can be seen in the Network Tab

  *Endpoint*: https://<instance>.atlassian.net/rest/api/3/issue/ABC-123/assignee
  *Status*: 400 (Bad request)
  Response: {"errorMessages":["We couldn't save ABC-123. Copy your content, then try reloading the issue."],"errors":{}}
  

Workaround

Currently, there is no known workaround for this behavior. A workaround will be added here when available. If you have the same symptoms on your instance, contact Atlassian support.