Autocompletes orders: No emails sent

add_filter( 'woocommerce_payment_complete_order_status', function() {
  return 'completed';
}, 10, 2 );

I’m basically using this function (simplyfied) to autocomplete orders which were payed with payment providers like Sofort and GiroSolutions so the status doesn’t stick to processing. But using this hook doesn’t work. The order gets set to ‘completed’ but no emails (neither to the admin nor to the customer) are sent.

If I remove the hook the order gets set to ‘processing’ and all emails get sent correctly.

What am I doing wrong here?

https://wordpress.org/plugins/woocommerce/