Confluence allows the embedding of links by simply using the regular link feature and clicking the "embed" icon (see last screenshot). One of the supported embed types is Jira.

When a page gets embedded in Confluence that has an embedded Jira link, Jira will refuse the connection since atlassian-dev.net is not in the allowed domains, therefore Forge apps cannot embed Jira within an Iframe.

In this feature request then, it would be useful for app developers and partners to have atlassian-dev.net to the list of allowed domains.