We can't see the content iframe displayed when the story is published
Summary
The problem is: we can't see the content iframe displayed in Composer when the story is published but we can see it since the edition story in Composer.
Procedure
You need to deselect the lazy-loading on your Article Body - Arc Block. All HTML Blocks from Composer won't load if that box is checked. Also this is not unique to iframes - this is about any content that requires loading in the Raw HTML block in Composer.