Customize some checkout fields properties in Woocommerce 3+

I have updated the placeholders for basket fields via the WooCommerce settings, as follows:

However on the frontend, the previous placeholders remain:

I have also used the following code, adapted from another StackOverflow thread which aims to override the placeholder content, but the defaults still persist.

add_filter('woocommerce_default_address_fields', 'override_default_address_checkout_fields', 20, 1);
function override_default_address_checkout_fields( $address_fields ) {
    $address_fields['state']['placeholder'] = 'State';
    $address_fields['postcode']['placeholder'] = 'Postcode';
    return $address_fields;
}

Please advise on how I can achieve the desired placeholder text!

In woocommerce default there is no settings like you describe. So you are using a third party plugin, to customize fields. So you need to try multiple ways with a higher hook priority.

So try those one by one:

1). Using woocommerce_default_address_fields filter hook (with a higher priority hook):

add_filter('woocommerce_default_address_fields', 'customize_default_address_fields', 10000, 1 );
function customize_default_address_fields( $address_fields ) {
    $address_fields['state']['placeholder'] = __('State', 'woocommerce');
    $address_fields['postcode']['placeholder'] = __('Postcode', 'woocommerce');
    return $address_fields;
}

2). Using woocommerce_checkout_fields filter hook (only for checkout):

add_filter('woocommerce_checkout_fields', 'customize_checkout_fields', 10000, 1 );
function customize_checkout_fields( $fields ) {
    $fields['billing']['billing_state']['placeholder'] = __('State', 'woocommerce');
    $fields['shipping']['shipping_state']['placeholder'] = __('State', 'woocommerce');
    $fields['billing']['billing_postcode']['placeholder'] = __('Postcode', 'woocommerce');
    $fields['shipping']['shipping_postcode']['placeholder'] = __('Postcode', 'woocommerce');
    return $fields;
}

3). Using woocommerce_billing_fields and woocommerce_shipping_fields filters hooks (for checkout and My Account addresses):

add_filter('woocommerce_billing_fields', 'customize_billing_fields', 10000, 1 );
function customize_billing_fields( $billing_fields ) {
    $billing_fields['billing_state']['placeholder'] = __('State', 'woocommerce');
    $billing_fields['billing_postcode']['placeholder'] = __('Postcode', 'woocommerce');

    return $billing_fields;
}

add_filter('woocommerce_shipping_fields', 'customize_shipping_fields', 10000, 1 );
function customize_shipping_fields( $shipping_fields ) {
    $shipping_fields['shipping_state']['placeholder'] = __('State', 'woocommerce');
    $shipping_fields['shipping_postcode']['placeholder'] = __('Postcode', 'woocommerce');

    return $shipping_fields;
}

All code goes in function.php file of your active child theme (or active theme).

I hope that one of those will work. Without a third party plugin each of those codes works perfectly.