Request Participants not addable, failing with empty error message due to other field validations

Summary

If the current assignee of a JIRA Service Desk issue does not have Assignable permissions, Customers cannot be added to the Participants field through the Customer Portal.

Steps to Reproduce

1. Create a request in a SD
2. Assign it to a user, use "agent"
3. Go to user and roles and remove "agent" as a service desk team member
4. Try to add any user as a request participant

Expected Results

• Request participant added.
• Assignee field should fail on its own and not blow up another field.

Actual Results

The error "Failed to update participants field, errors: " occurs and the field is not updated as per the attached screenshot.

Workaround

Provide the assignee with the correct permissions. They may need to be unassigned / reassigned the issue for this to work.

Notes

This issue should be reproducible by making any other field fail to update while trying to add request participants, the steps above are just the way it was discovered in a support case.