Embedding Scribe slides iframe in SharePoint - HTML Field Security set up but still blocked!

I'm running into an issue with embedding Scribe slides in SharePoint:

I’ve added http://scribehow.com to the HTML Field Security settings at the site collection level under "Allow contributors to insert iframes only from the following domains."

I confirmed that DenyAddAndCustomizePages is disabled

I tested using this Embed Web Part (sort of)

<iframe src="https://scribehow.com/page-embed/blah_blah_blah" width="100%" height="640" allowfullscreen frameborder="0"></iframe>

The Embed Web Part shows an error saying the domain isn’t allowed:

"Embedding content from this website isn't allowed, but your admin can change this setting by adding http://scribehow.com to the list of sites that are allowed."

I've also:

• Tested after a couple hours for propagation
• Verified that other iframes (e.g., YouTube) work fine in the same web part
  • Disabled Youtube in the HTML Field Security settings, it still works???
• Cleared the browser cache and tested in multiple browsers

Despite all this, the embed still throws that error, so....

• Is there something I’m missing in configuring HTML Field Security or tenant-level settings?
• Has anyone successfully embedded Scribe in SharePoint, and if so, how did you configure it?

Any advice would be greatly appreciated!