Seek Bar thumb not transparent in Android 5.0 API 21 Lollipop

API Level < 21

Thumb is transparent, as expected.

On API Level 21

Thumb is opaque, Wonder why.

Here is the code

 <SeekBar
                android:id="@+id/ui_audioplayer_progressbar"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:progressDrawable="@drawable/custom_progress_bar_horizontal"
                android:thumb="@drawable/stud_slider" />

Interestingly, this problem can be seen and reproduced in the xml preview in eclipse itself, when you change the API Level perspectives.

Note:

• I run the same apk on both devices.
• Both devices are of the same make and model, i.e Nexus 5.
• The thumb image is a PNG with a transparent background, as apparent from the screenshot for API Level < 21

Is this a known bug in Lollipop or am i doing something wrong?

<SeekBar
    ....
    android:splitTrack="false" 
/>

For all API levels,

<Seekbar
    .....
    android:background="@null"
    android:splitTrack="false"
    ....
 />

will work. It worked for me.