Rails 5 test doesn't reproduce actual behaviour on PATCH

I'm really stuck here. I have a Language model, that gets updated in this method:

  def update
    @language = Language.find(params[:id])

    if @language.update_attributes(language_params)
      flash[:success] = 'Language information updated.'
      redirect_to @language
    else
      @skill_errors = @language.errors
      render 'edit'
    end
  end

The intended behaviour for a successful update reproduces when I run it on my local server, object gets updated, flash appears and redirect to @language happens.

In the test, however, I only get the 200: Success response, and the object doesn't get updated. Here's the test code:

  test 'should allow update when logged in as admin user' do
    sign_in(@admin)
    patch language_path(@ruby_language), params: { language: { name: 'Test'} }
    assert_not flash.empty?
    assert_redirected_to @ruby_language
    @ruby_language.reload
    assert_equal 'Test', @ruby_language.name
  end

@admin_user and @ruby_language are defined in fixtures. All the asserts in this test fail, including the last one, with reload. My guess is that there might be some routing glitch caused by my usage of Devise and/or Kaminari gems? On the other hand, my Language routes are very simple: resources :languages, concerns: :paginatable (the concern is here for human-readable URL formatting). Please, keep in mind that everything works as intended, only tests fail for some reason... Thanks in advance!