Hi,
We added some custom fields in outgoing email notification with issue created event by following the below procedure shared by you but we can’t see those custom fields in outgoing email notification after issue getting created. please check the attached snapshot of notification email.
we followed the URL shared the Atlassian vendor.
https://developer.atlassian.com/server/jira/platform/adding-custom-fields-to-email/
please review and provide us a solution and reason for not working.