I think this is a bug, but I can't seem to embed iframes in the rich text editor without the caption message "Type image caption here (optional)" appearing with it. When I try to delete any character in that phrase, the entire embed disappears. It's weird! I've tried editing in the CMS as well, same thing.
Here's an example: http://hell-toupee.webflow.io/blog/map-of-snow-shoe