Coordinator work with RecyclerView 'A' and RecyclerView 'B' in RecyclerView 'A'

Question

coordinatorlayout work with recyclerview 'A' and another recyclerview 'B' in recyclerview 'A'. recyclerview 'B' as a item in recyclerview 'A'

See above images.

If scroll UI start from 'Part A' . It's ok. Toolbar hide or show.

If scroll UI start from 'Part RecyclerView'. Toolbar's stuck.

Why???

Edit01 :

  <android.support.design.widget.AppBarLayout
      android:id="@+id/appbar_layout"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:fitsSystemWindows="true">

      <android.support.v7.widget.Toolbar
          android:id="@+id/toolbar"
          android:layout_width="match_parent"
          android:layout_height="?attr/actionBarSize"
          android:background="#fefefe"
          android:minHeight="?attr/actionBarSize"
          app:contentInsetLeft="0dp"
          app:contentInsetStart="0dp"
          app:layout_scrollFlags="scroll|enterAlways">
          <!-- custom layout -->
      </android.support.v7.widget.Toolbar>

  </android.support.design.widget.AppBarLayout>
  <android.support.v4.view.ViewPager
      android:id="@+id/viewpager"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:scrollbars="none"
      app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

ViewPager contains Fragment , and the Fragment contains RecyclerView. Let's show you the item of RecyclerView

  <RelativeLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content">

            // Part A

      <RelativeLayout
          android:id="@+id/rl_artist_info"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:padding="@dimen/item_card_artist_margin">
       </RelativeLayout>

       //Part RecyclerView
       <android.support.v7.widget.RecyclerView
           android:id="@+id/rv_artist_product"
           android:layout_width="match_parent"
      android:layout_height="@dimen/item_card_artist_products_height"
           android:layout_below="@id/rl_artist_info"
           android:scrollbars="none"/>
   </RelativeLayout>

If scroll UI start from 'Part A' . It's ok. Toolbar hide or show.
If scroll UI start from 'Part RecyclerView'. Toolbar's stuck.