Checkout forgets delivery method (custom carrier)

Like the title stated.
We’ve created a custom shipping module, it’s quite basic and works like it should.
However, when I continue to the checkout it seems to forget it and select another shipping method. If I select it again and refresh it also forgets it.
It doesn’t matter if the user is logged in or a guest etc.. There are no (error)logs in the console nor in var/log/....

What am I missing here?
Why doesn’t it keept the correct delivery method?

(Renamed vendor/module since it contains the company their name)
Carrier code
Vendor/Module/Model/Carrier/ShippingBezorging.php

<?php
/**
 * Copyright © 2021 .... All rights reserved.
 */
namespace VendorModuleModelCarrier;

use MagentoQuoteModelQuoteAddressRateRequest;
use MagentoShippingModelRateResult;

class ShippingBezorging extends MagentoShippingModelCarrierAbstractCarrier implements
    MagentoShippingModelCarrierCarrierInterface
{
    /**
     * @var string
     */
    protected $_code = 'bezorging';

    /**
     * @var MagentoShippingModelRateResultFactory
     */
    protected $_rateResultFactory;

    /**
     * @var MagentoQuoteModelQuoteAddressRateResultMethodFactory
     */
    protected $_rateMethodFactory;

    /**
     * Shipping constructor.
     *
     * @param MagentoFrameworkAppConfigScopeConfigInterface          $scopeConfig
     * @param MagentoQuoteModelQuoteAddressRateResultErrorFactory  $rateErrorFactory
     * @param PsrLogLoggerInterface                                    $logger
     * @param MagentoShippingModelRateResultFactory                  $rateResultFactory
     * @param MagentoQuoteModelQuoteAddressRateResultMethodFactory $rateMethodFactory
     * @param array                                                       $data
     */
    public function __construct(
        MagentoFrameworkAppConfigScopeConfigInterface $scopeConfig,
        MagentoQuoteModelQuoteAddressRateResultErrorFactory $rateErrorFactory,
        PsrLogLoggerInterface $logger,
        MagentoShippingModelRateResultFactory $rateResultFactory,
        MagentoQuoteModelQuoteAddressRateResultMethodFactory $rateMethodFactory,
        array $data = []
    ) {
        $this->_rateResultFactory = $rateResultFactory;
        $this->_rateMethodFactory = $rateMethodFactory;
        parent::__construct($scopeConfig, $rateErrorFactory, $logger, $data);
    }

    /**
     * get allowed methods
     * @return array
     */
    public function getAllowedMethods()
    {
        return [$this->_code => $this->getConfigData('name')];
    }

    /**
     * @return float
     */
    private function getShippingPrice($configPrice)
    {

        $shippingPrice = $this->getFinalPriceWithHandlingFee($configPrice);

        return $shippingPrice;
    }

    /**
     * @param RateRequest $request
     * @return bool|Result
     */
    public function collectRates(RateRequest $request)
    {
        if (!$this->getConfigFlag('active')) {
            return false;
        }

        /** @var MagentoShippingModelRateResult $result */
        $result = $this->_rateResultFactory->create();

        $configPrice = false;
        $country = $request->getDestCountryId();
        $areas = json_decode($this->getConfigData('postal_areas'));

        if($country!=='NL'){
            $error = $this->_rateErrorFactory->create();
            $error->setCarrier($this->_code);
            $error->setCarrierTitle($this->getConfigData('title'));
            $error->setErrorMessage(
                empty($country)||$country=='US'?'Vul de gegevens hierboven in om te kijken of we bezorgen op jouw adres.':'Onze eigen bezorging is niet beschikbaar buiten Nederland.'
            );
            return $error;
        }
        $postalcode = $request->getDestPostcode();
        if(!empty($postalcode)){
            $re = '/([0-9]+)/m';
            preg_match_all($re, $postalcode, $matches, PREG_SET_ORDER, 0);
            if(empty($matches))
                return false;
            $postalcode = intval($matches[0][0]);
            foreach($areas as $area){
                $_country = is_array($area->country)? $area->country[0] : $area->country;
                if($country != $_country)
                    continue;
                $postals = explode('-', $area->postalcode);
                if(count($postals) > 1){
                    $start = intval(str_replace('*', '0', $postals[0]));
                    $end = intval(str_replace('*', '9', $postals[1]));
                    if($postalcode > $start && $postalcode < $end){
                        $configPrice = $area->price;
                        break;
                    }
                }
                else{
                    $postal = str_replace('*', '[0-9]', $postals[0]);
                    $re = '/' . $postal . '/m';
                    preg_match_all($re, $postalcode, $matches, PREG_SET_ORDER, 0);
                    if(!empty($matches)){
                        $configPrice = $area->price;
                        break;
                    }
                }
            }
        }else
            $configPrice = 20; // Added for testing purpose
        if($configPrice===false){
            $error = $this->_rateErrorFactory->create();
            $error->setCarrier($this->_code);
            $error->setCarrierTitle($this->getConfigData('title'));
            $error->setErrorMessage(
                'Eigen bezorging is niet beschikbaar voor jouw postcodegebied.'
            );
            return $error;
        }

        /** @var MagentoQuoteModelQuoteAddressRateResultMethod $method */
        $method = $this->_rateMethodFactory->create();

        $method->setCarrier($this->_code);
        $method->setCarrierTitle($this->getConfigData('title'));
        // same carrier
        // with different method
        $method->setMethod($this->_code);
        $method->setMethodTitle($this->getConfigData('name'));

        $amount = $this->getShippingPrice($configPrice);

        $method->setPrice($amount);
        $method->setCost($amount);

        $result->append($method);

        return $result;
    }
}