URL for Single Image Field not working with wp filter

Hello

I'm trying to create the option for an alternative featured image.

This basically works fine with this filter as described here
https://wordpress.stackexchange.com/questions/299844/possible-to-swap-in-a-placeholder-image-globally-for-the-post-thumbnail-url-if-o

But it only works when I hardcode the URL. As soon I want it to fetch from single image field, I got plenty of errors. Although only fetching the url by it self is working fine.

See example in this short video (including audio)
https://www.berrycast.com/conversations/86c539fb-e04e-57b5-91a1-6f38faddc18a

I spent already many hours without success and hope for a hint, why this is not working.
Thanks
François

Filtercode

function gof_image_src( $image, $attachment_id, $size, $icon ) {

    if( ! is_single() ) { return $image; }

    $opt = rwmb_meta( 'gop_base_bg_img_opt' );

    if( $opt == 'show-global' ) {

        $img_alt = rwmb_meta( 'gop_base_img', ['size' => 'full'] );
        $img_alt_url = $img_alt['url'];
        $image = [$img_alt_url];
        //$image = ['https://dev-mf.local/wp-content/uploads/demo-po-tasha-jolley-jqSysL9rQHc-unsplash.jpg',];
    }

    return $image;
}
add_filter( 'wp_get_attachment_image_src', 'gof_image_src', 10, 4 );