Android problem with nine-patch png elevation profile

Question

I am working with an ui showing chat records where needs message bubbles(for containing message text),I konw i shoud using 9-patch png and so i did,but i got an confusing problem.[Click here to view image][1] https://i.stack.imgur.com/qK2FY.jpg As you see,the right bubble has a normal elevation as expected but the left not but with a rectangle profile.The problem is that the left one is just mirrored from the right one(I have adjust the 9-patch reference lines).So what wrong with that? There is another clue that when i replace the left bubble png with the right bubble png,the problem is still there.The following is my layout xml.

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:gravity="end"
    android:orientation="horizontal">


    <de.hdodenhof.circleimageview.CircleImageView
        android:id="@+id/iv_avatar"
        android:layout_width="@dimen/d_fab_mini"
        android:layout_height="@dimen/d_fab_mini"
        android:layout_alignParentEnd="true"
        android:layout_marginTop="@dimen/spacing_normal"
        tools:src="@drawable/avatar_default" />

    <TextView
        android:id="@+id/tv_message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignTop="@+id/iv_avatar"
        android:layout_marginTop="@dimen/spacing_small"
        android:layout_marginEnd="@dimen/spacing_normal"
        android:layout_marginBottom="@dimen/spacing_normal"
        android:layout_toStartOf="@+id/iv_avatar"
        android:autoLink="web"
        android:background="@drawable/bubble_sent"
        android:elevation="@dimen/z_card"
        android:gravity="center_vertical"
        android:letterSpacing="@fraction/letter_spacing_message"
        android:paddingStart="@dimen/spacing_large"
        android:paddingTop="@dimen/spacing_small"
        android:paddingEnd="@dimen/spacing_large"
        android:paddingBottom="@dimen/spacing_small"
        android:textColor="@android:color/white"
        tools:text="你好，我是XJUNZ" />

</RelativeLayout>