Atlassian Update:

The functionality has been disabled to prevent sites from changing the email address while the sites transition to new email servers.
The functionality will be available in a couple of weeks with additional checks to verify that the domain is valid and DKIM/SPF records are in place to use an external domain.

Find more details in https://confluence.atlassian.com/adminjiracloud/configuring-jira-cloud-to-send-emails-on-behalf-of-your-domain-900996548.html

Summary

The project email address setting is not visible and it prevents users from changing the default jira@instancename.atlassian.net email to a custom email and vice versa.

Steps to reproduce

1. Go to cogwheel > Projects > select any project
2. Click on Notifications
3. Observe that the default email notification setting is not visible:
   

Expected behavior

Users should able to edit the default email notification setting:

Actual behavior

Default email notification setting is not visible