Checkout page: cannot make zip code and city field to appear next to each other

I imported your Megamarket demo.
I applied regular WooCommerce snippet code in order to adapt the billing fields form on the checkout page:

add_filter( 'woocommerce_checkout_fields', 'customize_billing_fields' );
function customize_billing_fields( $fields ) {
    // Remove billing_address_2 field
    if ( isset( $fields['billing']['billing_address_2'] ) ) {
        unset( $fields['billing']['billing_address_2'] );
    }

	// Remove billing_state field
    if ( isset( $fields['billing']['billing_state'] ) ) {
        unset( $fields['billing']['billing_state'] );
    }
	
    // Make billing_phone field optional
    if ( isset( $fields['billing']['billing_phone'] ) ) {
        $fields['billing']['billing_phone']['required'] = false;
    }

	// Display billing_postcode and billing_city fields in the same row
    if ( isset( $fields['billing']['billing_postcode'] ) && isset( $fields['billing']['billing_city'] ) ) {
        $fields['billing']['billing_postcode']['class'] = array( 'form-row-first' );
        $fields['billing']['billing_city']['class'] = array( 'form-row-last' );
    }
	
    return $fields;
}

My goal:
Having the zip code field and city field to appear next to each other.

Actual result:
Despite the snippet both fields still have 100% width each. I assume because of your theme.

I recorded a screencast for you, see attached.
Zip code field = “Postleitzahl”, city field = “Stadt”
Very strange: You will see in second 00:04, for half a second it is visible that the zip code field and city field appear next to each other.