$_POST is empty on saved_post hook

Question

I'm developing a Wordpress website using astra-child and having issue in getting Form Data. I'm trying to get my custom field in saved_post hook but the $_POST is empty and the fields in the metadata are always the previous.

I`m using acf plugin and tried the hook 'acf/saved_post' which is not even triggering. also tried some other wp hooks like publish_post , rest_after_insert_post

The action in functions.php

  add_action("save_post", "on_saved_post", 999999);

function on_saved_post($post_id)
{
    $json_string_data = file_get_contents('php://input');
    $decoded_data = json_decode($json_string_data, true);
    write_log("json " . print_r($decoded_data, true));
    write_log("post " . print_r($_POST, true));
    write_log("get" . print_r($_GET, true));
    write_log("req" . print_r($_REQUEST, true));
}

My Output

json Array
(
    [id] => 9004111222328892
    [content] => 
)

post Array
(
)

getArray
(
    [_locale] => user
)

reqArray
(
    [_locale] => user
)

Any ideas?