why Knowledge Article rich text removes iframe with sandbox url containing double "--"

Question

let say you have a static public resource https://myDomain--sandboxName.sandbox.my.salesforce-sites.com/resource/fileName

now try to include this into a knowledge Article rich text as an image and an iframe tags

here is a snipped iframe

<iframe src="https://myDomain--sandboxName.sandbox.my.salesforce-sites.com/resource/fileName" frameborder="1" height="500" scrolling="yes" width="500" />

you may see that the image tag is saved into the knowledge article but the iframe is removed, I bring this case because some times there is a requirement for exposing a form within an article from a public site.

I found this while reviewing the effect of enhanced domains into articles, if you use the old url format https://sandbox-myDomain.InstanceName.force.com/SiteName/VFPage this will work fine even though the browser is redirected to the new url format https://myDomain--sandboxName.sandbox.my.salesforce-sites.com/SiteName/VFPage