How to filter products by multiselect attribute in GraphQL request?

So, I have a custom product attribute:

$eavSetup->addAttribute('catalog_product', 'visible_in_country', [
    'type' => 'varchar',
    'backend_type' => 'text',
    'backend_model' => MagentoEavModelEntityAttributeBackendArrayBackend::class,
    'input' => 'multiselect',
    'searchable' => true,
    'filterable' => true,
    'filterable_in_search' => false,
    ...
]);

In Elasticsearch, the values of the attribute are saved as a string and as an array:

{ ... ,"hits":[{ ... ,"_source":{ ... ,"sku":"test-sku", ... "visible_in_country":["DE","UK"], ... }},{ ... ,"_source":{ ... ,"sku":"test-sku-1", ... "visible_in_country":"UK" ... }}]}}

This is how I rewrite the product resolver to filter by country (which is obviously wrong):

<?php
declare(strict_types=1);

namespace TestCountryBasedCatalogForGraphQLPlugin;

class ProductResolverAlignOutput
{
    public function __construct(
        protected MagentoFrameworkAppRequestInterface $request
    ) {
    }

    public function beforeResolve(
        MagentoCatalogGraphQlModelResolverProducts $subject,
        MagentoFrameworkGraphQlConfigElementField $field,
        MagentoFrameworkGraphQlQueryResolverContextInterface $context,
        MagentoFrameworkGraphQlSchemaTypeResolveInfo $info,
        array $value = null,
        array $args = null
    ): array {
        if ($country = $this->request->getHeader('country')) {
            // $country = 'DE'
            $args['filter']['visible_in_country'] = ['eq' => $country];
        }
        return [$field, $context, $info, $value, $args];
    }
}

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="MagentoCatalogGraphQlModelResolverProducts">
        <plugin name="ProductResolverAlignOutput"
                type="TestCountryBasedCatalogForGraphQLPluginProductResolverAlignOutput"
                sortOrder="1" disabled="false" />
    </type>
</config>

This is the GraphQL query:

query {
    products {
        total_count
        items {
            name
            sku
            visible_in_country
            saleable_in_country
            stock_status
        }
    }
}

The result always contains 0 items.

How to rewrite the resolver to make filtering by country work? Or maybe I need to update search_request.xml?