Darker triangle area appears at corner of transparent and rounded buttons

Issue

I was tried to set border radius on buttons by drawable XML files to create rounded corner buttons, but when use them as Android background for buttons I saw darker parts on the corners like triangle!

Image: https://i.stack.imgur.com/CoE5r.jpg

Who knows how these areas can be transparent?

XML drawable code :

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item>
      <shape
        android:shape="rectangle">
            <stroke android:width="1dp" android:color="#c3000000" />
            <solid android:color="#6e000000" />
          <corners android:radius="20dp" />
        </shape>
   </item>
</layer-list>

Solution

You are right! this problem was only in Android Stuido preview, Thanks for you answer and guidance !

Answered By - igr33dy