single image field with the id _thumbnail_id is not showing after latest updates

Hi,

Do you have featured image enable for that post type? If yes, then maybe there are 2 fields with the same ID and the default featured image might override the Meta Box field.

Hi,

After removing the default featured image and use the single image field with the ID _thumbnail_id to save value for this meta key, it works as well with the latest version of Meta Box v5.2.10. Please try again with my sample code

// Remove the featured image
add_action( 'init', function () {
    remove_post_type_support( 'post', 'thumbnail' );
} );

add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
    // Get the current post thumbnail and set as the default value for the single image field.
    $post_id = filter_input( INPUT_GET, 'post', FILTER_SANITIZE_NUMBER_INT );
    $post_thumbnail = get_post_meta( $post_id, '_thumbnail_id' );

    $meta_boxes[] = [
        'title'      => 'Featured image',
        'post_types' => 'post',
        'fields'     => [
            // Register a single image field
            [
                'type' => 'single_image',
                'id'   => '_thumbnail_id', // This is the must!
                'name' => 'Fake featured image',
                'std'  => $post_thumbnail,
            ],
        ],
    ];

    return $meta_boxes;
} );

Let me know how it goes.