FormRequest does not work in same way as \Illuminate\Http\Request in unit testing?

[ProteanCode]

Consider following versions of controller methods
public function step1(\App\Http\Requests\RegistrationStep1 $request)
and just
public function step1(\Illuminate\Http\Request $request)

the second one works fine with following unit test

public function testRegistrationSteps() {
    $users = User::count();

    $this->followingRedirects()->post(route('step1'), [
        'data' => '...'
    ])->assertStatus(200);

    $this->assertAuthenticated();
    $this->assertGreaterThan($users, User::count());
}

but the first definition that uses FormRequest does not pass, both assertion fails.

[ProteanCode]

I think I may report this as a bug because I have created following class:

class RequestX extends \Illuminate\Http\Request {}

and the unit test ends with failure when I use RequestX as the type of $request. This should not happen

Laravel 8.16 PHP 7.4 if that matters