meta clave es testsin embargo, es un objeto.

Ejemplo:

$data = get_post_meta($post_id, "test", true);
echo $data["a"];
echo $data["b"];
echo $data["c"];

👉 Pregunta:

• Cómo ordenar datos por meta personalizado (en forma de objeto) en la lista de publicaciones de WordPress.
• me gustaría ordenar A, B y C columnas

Ref: wp_meta_query

<!-- Post Editor | custom field in MetaBox -->
<input name="test[a]" />
<input name="test[b]" />
<input name="test[c]" />
<button type="submit">Submit</button>

add_filter("manage_customPost_posts_columns", function($col) {
    $col["a"] = "A";
    $col["b"] = "B";
    $col["c"] = "C";
    return $col;
});

//________________________________________________________

add_filter("manage_edit-customPost_sortable_columns", function($col) {
    $col["a"] = "a";
    $col["b"] = "b";
    $col["b"] = "c";
    return $col;
});

//________________________________________________________

add_action("pre_get_posts", function($query) {
    $orderby = $query->get("orderby");

        // ▼▼▼ Issue start from here ▼▼▼
        $meta_query = array(
            "key" => "test[".$orderby."]",
        );
        $query->set("meta_query", $meta_query);
        $query->set("orderby", "value");
        // ▲▲▲ Issue end from here ▲▲▲

});