Hi Toufiq,
appreciate your effort/answer - but it doesn´t really help me :)
The problem is, that the following iframe html code gets stripped:
<p><iframe src="/https://player.vimeo.com/video/1059666478?portrait=0&title=0&byline=0" width="720" height="405" style="display: block; margin-left: auto; margin-right: auto; vertical-align: center;" allowfullscreen="allowfullscreen"></iframe></p>
I have placed this exact code inside a native Joomla 5 article, plus in a FABRIK record, and even in a RAWhtml field in a SPPB page - and it works perfectly in all these instances.
And I've checked the settings in the JCE editor, and no "Clean Up HTML" is set, and iFrames are not prohibitted.
But, when the html string is placed in the rich text field in Dynamic Content, it gets stripped. 
At first it shows, but after being published, it is stripped, and when returning to the rich text field, the only html present in the field is <p></p>.
Could I get you to take a look at this?
kr./John