WooCommerce - Place Orders On Hold if billing and shipping information are different

functions.php

<?php

/* PLACE ORDERS ON HOLD WHEN BILLING & SHIPPING INFO DO NOT MATCH */
add_action( 'woocommerce_thankyou', 'woocommerce_billing_shipping_address_match', 10, 1);
function woocommerce_billing_shipping_address_match( $order_id ) {
    if ( ! $order_id ) {
        return;
    }
    // Get the order id
    $order = wc_get_order( $order_id );

    // Customer billing information details
    $billing_first_name = $order->get_billing_first_name();
    $billing_last_name = $order->get_billing_last_name();
    $billing_company = $order->get_billing_company();
    $billing_address_1 = $order->get_billing_address_1();
    $billing_address_2 = $order->get_billing_address_2();
    $billing_city = $order->get_billing_city();
    $billing_state = $order->get_billing_state();
    $billing_postcode = $order->get_billing_postcode();
    $billing_country = $order->get_billing_country();

    // Customer shipping information details (from account)
    $shipping_first_name = $order->get_shipping_first_name();
    $shipping_last_name  = $order->get_shipping_last_name();
    $shipping_company    = $order->get_shipping_company();
    $shipping_address_1  = $order->get_shipping_address_1();
    $shipping_address_2  = $order->get_shipping_address_2();
    $shipping_city       = $order->get_shipping_city();
    $shipping_state      = $order->get_shipping_state();
    $shipping_postcode   = $order->get_shipping_postcode();
    $shipping_country    = $order->get_shipping_country();

    // check if any the billing info does not match the shipping info and place on hold if not
    if($billing_first_name !== $shipping_first_name || $billing_last_name !== $shipping_last_name || $billing_address_1 !== $shipping_address_1 || $billing_address_2 !== $shipping_address_2 || $billing_city !== $shipping_city || $billing_state !== $shipping_state || $billing_postcode !== $shipping_postcode || $billing_country !== $shipping_country) {
            // Place the order on hold
            $order->update_status( 'on-hold' );
    }
}

Here you go: https://gist.github.com/charliesjc/b3bfa1e2c35bd3c21f426c05b1db8234.
So I would add this:
if ($order_status == 'processing') { $order_status = 'on-hold'; }
in your last if statement and then return the $order_status at the end of the function. The only real difference to your original code is the hook.

Your link text and link don't match. This will work:

https://gist.github.com/charliesjc/b3bfa1e2c35bd3c21f426c05b1db8234

I know. I even said so in my previous comment. I haven't got a clue why it's doing that. The link that's in the message is exactly the same as the text, but it's ignoring it completely and linking back to this gist.