Tax/VAT field configuration is not working on multistore Magento2 Core

Question

I have a multistory website as Retail-Website and Wholesale-Website

Configuration with fresh Magneto2.2 at Store >> Configuration >> Customer Configuration >> Show Tax/VAT Number

Retail-Website

Wholesale-Website

When I check the front-end of customer registration the configuration dint apply I did cache clean, I tried different combination of the setup dint work.

In Table customer_eav_attribute_website I can see the configuration is correctly added.

I tried this in version 2.2 and in 2.5 fresh project.

Scenarios 1

Admin Input

    webA >> required

    webB >> optional

Store output

    webA >> required

    webB >> required

After flush cache Store output

    webA >> optional

    webB >> optional

Scenarios 2

Admin Input

    webA >> required

    webB >> required

Store output

    webA >> required

    webB >> required

After flush cache Store output

    webA >> required

    webB >> required

Scenarios 3

Admin Input

    webA >> optional

    webB >> required

Store output

    webA >> optional

    webB >> optional

After flush cache Store output

    webA >> optional

    webB >> optional

Answer 1

This is bug. Try following way to fix:

app/code/SR/MagentoCommunity/etc/frontend/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Checkout\Block\Checkout\AttributeMerger">
        <plugin name="sr_vat_id_fix"
                type="SR\MagentoCommunity\Plugin\Checkout\Block\Checkout\AttributeMerger" sortOrder="1"/>
    </type>
</config>

app/code/SR/MagentoCommunity/Plugin/Checkout/Block/Checkout/AttributeMerger.php

<?php
namespace SR\MagentoCommunity\Plugin\Checkout\Block\Checkout;

use Magento\Store\Model\ScopeInterface;
use Magento\Framework\App\Config\ScopeConfigInterface;
use Magento\Store\Model\StoreManagerInterface;

class AttributeMerger
{
    /**
     * @var ScopeConfigInterface
     */
    private $scopeConfig;

    /**
     * @var StoreManagerInterface
     */
    private $storeManager;

    /**
     * AttributeMerger constructor.
     *
     * @param ScopeConfigInterface $scopeConfig
     * @param StoreManagerInterface $storeManager
     */
    public function __construct(
        ScopeConfigInterface $scopeConfig,
        StoreManagerInterface $storeManager
    ) {
        $this->scopeConfig = $scopeConfig;
        $this->storeManager = $storeManager;
    }

    public function afterMerge(
        \Magento\Checkout\Block\Checkout\AttributeMerger $subject,
        $fields
    ) {
        foreach ($fields as $field => $fieldConfig) {
            if ($field == 'vat_id') {
                $store = $this->storeManager->getStore();
                $taxvatShow = $this->scopeConfig->getValue(
                    'customer/address/taxvat_show',
                    ScopeInterface::SCOPE_STORE,
                    $store->getId()
                );
                if ($taxvatShow == 'req') {
                    $fields[$field]['validation'] = array_merge(['required-entry' => true], $fields[$field]['validation']);
                } else {
                    $fields[$field]['validation'] = array_merge(['required-entry' => false], $fields[$field]['validation']);
                }
            }
        }

        return $fields;
    }
}