What is $context in wp_kses_allowed_html()?

Thanks @gappiah George. That is definitely a hint in the right direction!

From your link I learned that “post” is the most permissive and changing the context to “post” fixed the issue and let me iframes finally render dynamically from ACF fields 🙂

Looks like the codex had some good notes about the contexts and what each is for, but those didn’t get incorporated into the the API docs, but got attached as a user comment:

// strips all html (empty array)
$allowed_html = wp_kses_allowed_html( 'strip' );

// allows all most inline elements and strips all block level elements except blockquote
$allowed_html = wp_kses_allowed_html( 'data' );

// very permissive: allows pretty much all HTML to pass - same as what's normally applied to the_content by default
$allowed_html = wp_kses_allowed_html( 'post' );

// allows a list of HTML Entities such as  
$allowed_html = wp_kses_allowed_html( 'entities' );

Super stoked this is working now!

Here is the code I ended up using for future reference:

add_filter( 'wp_kses_allowed_html', 'acf_add_allowed_iframe_tag', 10, 2 );
function acf_add_allowed_iframe_tag( $tags, $context ) {
    if ( $context === 'post' ) {
        $tags['script'] = array(
            'src'             => true,
            'height'          => true,
            'width'           => true,
            'frameborder'     => true,
            'allowfullscreen' => true,
        );
    }

    return $tags;
}

`