Support Forum

You may be able to filter the WordPress featured image ID before it ever reaches the plugin or theme:

add_filter( 'get_post_metadata', 'tu_override_featured_image_id', 100, 4 );

function tu_override_featured_image_id( $metadata, $object_id, $meta_key, $single ) {
    if ( isset( $meta_key ) && 'thumbnail_id' == $meta_key ){
        remove_filter( 'get_post_metadata', 'tu_override_featured_image_id', 100 );
        $new_id = get_post_meta( $object_id, 'imatge_destacada_alternativa', true ) ;
        add_filter( 'tu_override_featured_image_id', 'getqtlangcustomfieldvalue', 100, 4 );

        if ( $new_id ) {
            $metadata = $new_id;
        }
    }

    return $metadata;
}

Not sure if it will work, but worth a shot.