WordPress: Append Text to Image Caption

Question

I am seeking help in appending text to image captions in WordPress. I have tried two different pieces of code without success. The image caption displays normally without the appended text.

I have tried the following:

add_filter('img_caption_shortcode', 'modify_caption_text', 10, 3);

function modify_caption_text($output, $attr, $content) {
    $caption = isset($attr['caption']) ? $attr['caption'] : '';
    $new_caption = $caption . ' - Custom Text';
    return str_replace($caption, $new_caption, $output);
}

and

function append_image_credit_to_caption($output, $attr, $content) {
    // Your additional text
    $additional_text = "<em>Custom Text</em>";

    // Append the additional text to the caption
    $output .= $additional_text;

    return $output;
}
add_filter('img_caption_shortcode', 'append_image_credit_to_caption', 10, 3);

As the code above is not returning any errors within WordPress, is there something I am overlooking?

Thank you