Custom Total Collector Not Calculating Shipping

I have a custom totals collector, which removes the VAT for exempt items. Here is the code:

app/code/Innox/VatExemption/etc/sales.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Sales:etc/sales.xsd">
    <section name="quote">
        <group name="totals">
            <item name="tax_subtotal" instance="VendorModuleModelSalesTotalQuoteSubtotal" sort_order="600"/>
        </group>
    </section>
</config>

app/code/Vendor/Module/Model/Sales/Total/Quote/Subtotal.php

<?php

namespace VendorModuleModelSalesTotalQuote;

use VendorModuleHelperData as ExemptionHelper;
use MagentoCustomerApiDataAddressInterfaceFactory as CustomerAddressFactory;
use MagentoCustomerApiDataRegionInterfaceFactory as CustomerAddressRegionFactory;
use MagentoTaxHelperData as TaxHelper;
use MagentoTaxApiDataQuoteDetailsInterfaceFactory;
use MagentoTaxApiDataQuoteDetailsItemInterfaceFactory;
use MagentoTaxModelSalesTotalQuoteTax;
use MagentoTaxModelConfig;
use MagentoTaxApiTaxCalculationInterface;
use MagentoTaxApiDataTaxClassKeyInterfaceFactory;
use MagentoFrameworkSerializeSerializerJson;

class Subtotal extends Tax
{
    protected $exemptionHelper;

    /**
     * Constructor
     * 
     * @param ExemptionHelper $exemptionHelper
     */
    public function __construct(
        Config $taxConfig,
        TaxCalculationInterface $taxCalculationService,
        QuoteDetailsInterfaceFactory $quoteDetailsDataObjectFactory,
        QuoteDetailsItemInterfaceFactory $quoteDetailsItemDataObjectFactory,
        TaxClassKeyInterfaceFactory $taxClassKeyDataObjectFactory,
        CustomerAddressFactory $customerAddressFactory,
        CustomerAddressRegionFactory $customerAddressRegionFactory,
        TaxHelper $taxData,
        Json $serializer,
        ExemptionHelper $exemptionHelper
    ) {
        $this->exemptionHelper = $exemptionHelper;

        parent::__construct(
            $taxConfig,
            $taxCalculationService,
            $quoteDetailsDataObjectFactory,
            $quoteDetailsItemDataObjectFactory,
            $taxClassKeyDataObjectFactory,
            $customerAddressFactory,
            $customerAddressRegionFactory,
            $taxData,
            $serializer
        );
    }

    /**
     * Calculate tax on product items. The result will be used to determine shipping
     * and discount later.
     *
     * @param MagentoQuoteModelQuote $quote
     * @param ShippingAssignmentInterface $shippingAssignment
     * @param AddressTotal $total
     * @return $this
     */
    public function collect(
        MagentoQuoteModelQuote $quote,
        MagentoQuoteApiDataShippingAssignmentInterface $shippingAssignment,
        MagentoQuoteModelQuoteAddressTotal $total
    ) {
        // return if no items in quote
        if (!$quote->getItems()) {
            return $this;
        }

        // set some default values
        $subtotal = 0;
        $tax = 0;
        $quoteId = $quote->getId();

        foreach ($quote->getItems() as $quoteItem) {
            // check for exempt item
            if ($this->exemptionHelper->isVatExemptProduct($quoteItem)) {
                // check for this item in current quote 
                if ($this->exemptionHelper->isExemptProductInQuote($quoteId, $quoteItem)) {
                    // remove VAT from item row price and subtotals
                    $this->setRowExclTax($quoteItem);
                }
            }   

            // add to cart subtotal
            $subtotal += $quoteItem->getBaseRowTotalInclTax();

            // add to cart tax
            $tax += $quoteItem->getBaseTaxAmount();

            $this->updateCartTotals($total, $subtotal, $tax);  
        }    

        return $this;
    }

    private function setRowExclTax($quoteItem) 
    {
        $rowTotal = ($quoteItem->getPrice() * $quoteItem->getQty());

        // calculate row total ex tax
        $quoteItem->setPrice($quoteItem->getPrice());
        $quoteItem->setBasePrice($quoteItem->getBasePrice());
        $quoteItem->setPriceInclTax($quoteItem->getPrice());
        $quoteItem->setBasePriceInclTax($quoteItem->getBasePrice());
        $quoteItem->setRowTotal($rowTotal);
        $quoteItem->setBaseRowTotal($rowTotal);
        $quoteItem->setRowTotalInclTax($rowTotal);
        $quoteItem->setBaseRowTotalInclTax($rowTotal);
        $quoteItem->setTaxAmount(0);
        $quoteItem->setBaseTaxAmount(0);
        $quoteItem->setTaxPercent(0);
    }

    private function updateCartTotals($total, $subtotal, $tax)
    {
        $total->setTotalAmount('subtotal', $subtotal);
        $total->setBaseTotalAmount('subtotal', $subtotal);
        $total->setSubtotalInclTax($subtotal);
        $total->setBaseSubtotalInclTax($subtotal);
        $total->setTotalAmount('tax', $tax);
        $total->setBaseTotalAmount('tax', $tax);
        $total->setGrandTotal($subtotal);
    }
}

but even with normal items (VAT inclusive) the shipping is not being calculated in the totals for the checkout page, e.g:

in this case the grand total should be £11.44

I’ve tried changing the sort order in the sales.xml file to 101 and 600 and a few other values in between but none of them seem to work for some reason. Any help would be greatly appreciated 🙂

Cheers