Magento – “Class mixed does not exist” with only one array param in the body in Magento 2

I have a custom Magento 2 PUT request that looks like:

<route method="PUT" url="/V1/items">
    <service class="Vendor\Items\Api\ItemsManagementInterface" method="replaceItems" />
        <resources>
            <resource ref="self" />
    </resources>
 </route>

Class interface:

/**
 * Description
 *
 * @param mixed $items
 * @return string
 */
public function replaceItems($items);

The request works with this JSON body payload:

{
  "items": [
    "url1",
    "url2"
  ],
  randomParam: true
}

Instead, an error is trown if the payload is just:

{
  "items": [
    "url1",
    "url2"
  ]
}

The full error returned from Magento REST Api:

{
"message": "Class mixed does not exist",
"code": -1,
"trace": "#0 vendor/magento/framework/Reflection/MethodsMap.php(155): ReflectionClass->__construct('mixed')
#1 vendor/magento/framework/Reflection/MethodsMap.php(106): Magento\Framework\Reflection\MethodsMap->getMethodMapViaReflection('mixed')
#2 vendor/magento/module-webapi/Controller/Rest/ParamsOverrider.php(203): Magento\Framework\Reflection\MethodsMap->getMethodsMap('mixed')
#3 vendor/magento/module-webapi/Controller/Rest/ParamsOverrider.php(147): Magento\Webapi\Controller\Rest\ParamsOverrider->isPropertyDeclaredInDataObject('GV\\Products\\Api…', 'replaceItems…', 'items')
#4 vendor/magento/module-webapi/Controller/Rest/InputParamsResolver.php(93): Magento\Webapi\Controller\Rest\ParamsOverrider->overrideRequestBodyIdWithPathParam(Array, Array, 'Vendor\\Items\\Api…', 'replaceItems…')
#5 vendor/magento/module-webapi/Controller/Rest.php(322): Magento\Webapi\Controller\Rest\InputParamsResolver->resolve()
#6 vendor/magento/module-webapi/Controller/Rest.php(239): Magento\Webapi\Controller\Rest->processApiRequest()
#7 vendor/magento/framework/Interception/Interceptor.php(58): Magento\Webapi\Controller\Rest->dispatch(Object(Magento\Framework\App\Request\Http))
#8 vendor/magento/framework/Interception/Interceptor.php(138): Magento\Webapi\Controller\Rest\Interceptor->___callParent('dispatch', Array)
#9 vendor/magento/framework/Interception/Interceptor.php(153): Magento\Webapi\Controller\Rest\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http))
#10 generated/code/Magento/Webapi/Controller/Rest/Interceptor.php(39): Magento\Webapi\Controller\Rest\Interceptor->___callPlugins('dispatch', Array, Array)
#11 vendor/magento/framework/App/Http.php(135): Magento\Webapi\Controller\Rest\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http))
#12 generated/code/Magento/Framework/App/Http/Interceptor.php(24): Magento\Framework\App\Http->launch()
#13 vendor/magento/framework/App/Bootstrap.php(256): Magento\Framework\App\Http\Interceptor->launch()
#14 index.php(39): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http\Interceptor))
#15 {main}"
}

I have already recompiled the DI and flushed the cache.

Also tried in the class interface: mixed[], string[], etc..

Goal: being able to make a request passing only a single array parameter, without the need to also pass a second param.