Embedding YouTube videos in an article doesn't work for portal-only customers

Issue Summary

If a KB article contains a video from YouTube, this will play fine in Confluence, but will not playback through the portal view of the customer.
Note: it works fine as the user is an Atlassian Account user, it doesn't if the user is a portal-only user or anonymous (non-login) customer.

Steps to Reproduce

1. Create a page in a Confluence space that is linked as a KB
2. Add a YouTube video to the page
3. View the article from the portal as a portal only user

Expected Results

The video should play fine in both Confluence and portal

Actual Results

It works in Confluence and in Jira for Atlassian Account users but not portal-only customers

Workaround

Embed the video using a Widget Connector