Unfortunately we were unable to reproduce your issue. We were able to add an iframe just fine. In our case, we did the following:
- Create a collection in Page Fragments.
- Add an iframe to our local URL (e.g. http://localhost) and publish it.
- Add the Page Fragment to a Page.
- Check the iframe in the site. In our case it worked.
If you did not follow these steps, could you let us know how you added iframes? This way we can try to reproduce the issue in the same way.
Please give detailed instructions on each step (e.g. Admin -> Sidebar -> Click on Page Fragments -> etc.).