Mention of user do not render for the full name on notification email if the outgoing notification email is modified.

Issue Summary

On email notifications, the users are occasionally and randomly receiving the AAID instead of the FULL name of the user. Although the setting of the outgoing email is configured to HTML, and the email is on HTML, the mention of the user isn't render by the email, and only the AAID is visible.

Steps to Reproduce

1. Mention a user in a comment of Jira issue (it will trigger a notification email)
2. Change the default outgoing notification email from the project settings >> notifications.

Expected Results

The mention of the user should come, in the email, rendered as the full name of the user

Actual Results

Although the email is on the HTML format, the mention of the user comes as the AAID:

Workaround

To stop the fallback to using the old email template, Change the default outgoing notification email from the project settings >> notifications to use the default domain name <tenant>.atlassian.net