Resolvi... Era bastante simples 😣
No caso alterei para...
function getPublishedPosts()
{
global $conn;
$sql = "SELECT p.*, u.username FROM posts AS p JOIN users AS u ON p.user_id=u.id WHERE p.published=? ORDER BY `created_at` DESC";
$stmt = executeQuery($sql, ['published' => 1]);
$records = $stmt->get_result()->fetch_all(MYSQLI_ASSOC);
return $records;
}