shortcode block calling an image showing in header in FSE themes

Hello, I meet a problem I cannot explain. I’ve created several shortcodes to insert content in single content and, if text through shortcodes shows at the good place, an image shows… before in the header, whatever I place the shortcode.
Tested on two different FSE themes.

Here is the code in the shortcode

// shortcode pour fse-alternative
function shortcode_fsealternative(){
global $post ;
        $alternativeimage = get_post_meta($post->ID, 'alternative_image', true);

        if (!empty($alternativeimage)) {
        $urlcmm='https://xxx/blog/wp-content/uploads/alternative/'.$alternativeimage;
        }
        else {
        $urlcmm='https:/xxx/blog/wp-content/uploads/images-site/click-x-640.png';
        if ( !is_single() ) {
        $urlcmm='https://xxx/blog/wp-content/uploads/images-site/click-x-home.png';}
        
        }
echo '<figure class="wp-block-post-featured-image"><img src="' . $urlcmm . '" /></figure>';
      }
add_shortcode('fsealternative', 'shortcode_fsealternative');