Radial Gradient does not work properly Pin

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Android

I work on an android application in Xamarin. For buttons background create e a xml. its so simple. the code is:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item>
    <shape android:shape="oval">
        <gradient   android:startColor="#ff000000" 
                    android:centerColor="#ffffffff" 
                    android:endColor="#00ffffff" 
                    android:type="radial" 
                    android:gradientRadius="100" 
                    />
    </shape>
</item>
</selector> 

when i user linear or sweep in android:type code everything is fine, a good transparent gradient appeare in my device( My device is HTC One M8). but when i user radial a bad solid color appear. no transparency. i want use this technique for the shadow under buttons. but if check the screenshot a very bad harshy solid color appear under the buttons.

is it a bug in the code or in my device or in xamarin deploy setting?

thanks for any help

ScreenShot