If launching a GNU screen with proc_open, subsequents proc_get_status always return (wrongly) running = false

$descriptorspec = array(
     0 => array("pipe", "r"),  // stdin
     1 => array("pipe", "w"),  // stdout
     2 => array("pipe", "w") // stderr
);
$p = proc_open('screen ...', $descriptorspec, $pipes);
var_dump(proc_get_status($p)['running']); // false (wrong)