The page you are redirected to will never contain _POST values. A form submission causes 2 server requests, the first to process the form, this includes the _POST values and the second to get the page you are redirecting to.
It may contain $_GET['thank-you'] but you may need to give your query parameter a value… I’m not sure
'return' => get_the_permalink(get_the_ID()).'?thank-you=1',
