I am trying to modify the caption of images in posts by filtering img_caption_shortcode, and then displaying the content using the_content(). I want to add bylines/credit on images, so I have created two fields in ACF that should be added inside the <figcaption> on all my posts. Basically I want to do what is description in the blog.

So from this:

<figcaption>
So this is a caption that describes the image.
</figcaption>

To this:

<figcaption class="post-thumbnail-caption">
So this is a caption that describes the image.
<span class="wp-block-image-byline">Photo: Gregor Gregson</span>
</figcaption>

I have tried just doing it as simple as possible, but I can not get it to work. Nothing happans/changes with the following code in my functions.php (I did first try the whole code from the example in the blog linked above).

add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 10, 3 );
 
function my_img_caption_shortcode( $output, $attr, $content ) {
    $output = 'test';
    return $output;
 
}

Any ideas on what I could be doing wrong?