wp_remote_post empty $_POST

Ok, I've searched a lot but cannot understand what I'm missing. I need to post some data to a php file on another server, but if I print the wp_remote_post variable I get:

{headers:{},body:,response:{code:200,message:OK} .....

here the code:

        $url  = 'https://xxx.domain.net/api/api.php';
    $args = array(
            'method'      = 'POST',
            'timeout'     = 45,
            'sslverify'   = false,
            'headers'     = array(
                    'Authorization' = 'xxxx',
                    'Content-Type'  = 'application/json; charset=utf-8',
            ),
            'body'        = json_encode($attachment_fields),
            'data_format' = 'body'
    );
    $request = wp_remote_post( $url, $args );

and $attachment_fields is an array built previously.

It seems that it sends it out (it says code 200) but in the php file the $_REQUEST (or $_POST) is empty... Sorry if is a stupid question but I'm struggling on it for a while..