PHP 7.4 & MySQL: caching_sha2_password Denying long (20c) passwords

Use this script to automatically set random length and content password, then reconnent to mysql server.

$mysqli = new mysqli('localhost', 'username', 'initialpassword', 'dbname');
$i=0;
while (true) {
    if($i!=0){
        $mysqli = new mysqli('localhost', 'username', $pw, 'dbname');
    }
    $pw = bin2hex(random_bytes(rand(1, 16)));
    $successd = $mysqli->query("ALTER USER 'username'@'localhost' IDENTIFIED WITH caching_sha2_password BY '{$pw}'");
    echo "times:{$i} pw:{$pw} {$successd}\n";
    $i++;
}

In my environment there's not a single access denied error occurred after iterate 1 million times.