If you have the “Eventbrite Tickets Extension: Additional Options” plugin installed. Try to deactivate the plugin and then update page in browser, the iframe should now show up. I confirmed it on one of my sites. I also noted that if the event were previously imported before update 4.5 it breaks. The solution for me was remove all imported events and reimport. Note that the new import doesn´t take original text formatting only text is imported and if you have any picture in your Eventbrite event it does not import either.