On a Full HTML field if I enter generic YouTube embed field using iframe it works fine:
<iframe width="560" height="315" src="https://www.youtube.com/embed/BZfB8Ny2Tkw" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
The iframe renders and displays video. But if I update the src attribute to something else, save the content and then render the page, the iframe is completely gone. I view source and it's not there. If I edit the page again the code is still in the body field.
If I set a debug breakpoint on say theme_preprocess_node I see the iframe content but at some point it's stripped out before rendering. A bit confused to say the least. This is D 9.5.4, CKEditor5.
I was expecting to see the iframe rendered.
The issue was the Blazy filter. Once I removed that all started working as expected.