In the old composer, after editing a html code you could see directly, if the embed is working and how it looks like. Now, in the new composer after editing, you still see the raw html code. You have to switch on manually to show a preview - then the preview is added, but the html code is still displayed, which is especially disturbing when it's a longer code.
Solution: Switch back to the old UX only see the code when editing it, preview, when not editing.
Simon Hrubesch
See attached screenshots, how it's implemented now, and how it was implemented previously