Service Desk creates a new issue instead of comment to existing issue for original email replied to by request participant

Summary

This is the same exact issue as https://jira.atlassian.com/browse/JSD-4058, which was initially reported with only Cloud cases and was thus resolved in Cloud, but not yet in Server.

When public signup is enabled in a Service Desk, and a customerA sends in an email while CC'ing customerB, an issue is correctly created.

When customerB replies to customerA's original email, customerB's new issue is created, rather than the reply being a comment on customerA's ticket.

Environment

JIRA 7.2.1
JIRA Service Desk 3.2.1

Steps to Reproduce

1. Create new service desk project and set it as open for public sign up
2. Setup email channel for the project
3. As customer A, send email to the sd email channel and cc customer B (A new issue will be created in Service Desk project)
4. As customer B, after receive the email from customer A, reply the email (Note this is the original email from A, not Service Desk generated email) with comment

Expected Results

Reply from B will be added as comment to the issue

Actual Results

A new issue has been created.