PathValidator::getUrlIfValid() does not support non-HTML/non-GET routes

had such issue, first request respects _format=json and next for the same server invocations are not.

after wasted hours of tracing and debugging I ended up with adding custom middleware

<?php

namespace Drupal\xxx;

use Drupal\Core\Http\InputBag;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\HttpFoundation\InputBag as SymfonyInputBag;

/**
 * FirstMiddleware middleware.
 */
class RequestFormatMiddleware implements HttpKernelInterface {

    /**
     * The kernel.
     *
     * @var \Symfony\Component\HttpKernel\HttpKernelInterface
     */
    protected $httpKernel;

    /**
     * Constructs the FirstMiddleware object.
     *
     * @param \Symfony\Component\HttpKernel\HttpKernelInterface $http_kernel
     *   The decorated kernel.
     */
    public function __construct(HttpKernelInterface $http_kernel) {
        $this->httpKernel = $http_kernel;
    }

    /**
     * {@inheritdoc}
     */
    public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {

		if(empty($request->getRequestFormat(null)) && !empty($request->get('_format'))) {
			$request->setRequestFormat($request->get('_format'));
		}

		Request::setFactory(
			function ($query, $request, $attributes, $cookies, $files, $server, $content) {
				$current = \Drupal::request();
				if(empty($attributes['_format']) && !empty($current->getRequestFormat(null))) {
					$attributes['_format'] = $current->getRequestFormat(null);
				}
				$request = new Request($query, $request, $attributes, $cookies, $files, $server, $content);
				foreach (['request', 'query', 'cookies'] as $bag) {
					if (!($bag instanceof SymfonyInputBag)) {
					$request->$bag = new InputBag($request->$bag->all());
					}
				}
				return $request;
			}
		  );

        return $this->httpKernel->handle($request, $type, $catch);
    }

}