The described behavior will occur because of config value KFAPI_CLEANSE_HTML_ANSWER_CONTENT set to 1 which is its default value. When enabled, Answer HTML text fields will be stripped of possibly malicious script tags like iframe, embed or Object that can contribute to cross-site scripting (XSS) attacks. Tag stripping will only occur before the Answer content is returned from the Knowledge Foundation API, and will not modify the data stored in the database.

So the data in the database will still be able to display the iframe content but when the same is fetched on the Customer Portal (CP) on the answer detail page using the KFAPI the above setting form KFAPI automatically strips the iframe tag and sends it back to CP. In order to not to strip set this value to 0 instead of its default value of 1 for all the sites created since 22A.