TransitionDrawable as background in TextView not working properly

Following the question: Animate the background color of a view on Android I'm trying to make a TextView disappear between two backgrounds through a TransitionDrawable resource. My transition XML file is as follows:

<?xml version="1.0" encoding="utf-8"?>
<transition xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/gray_bar" />
    <item android:drawable="@drawable/green_bar" />
</transition>

And I'm trying to set it as background at runtime:

TextView answerBox = (TextView) findViewById(R.id.answerBox);
answerBox.setBackgroundResource(R.drawable.correct_transition);
TransitionDrawable transition = (TransitionDrawable) answerBox.getBackground();
transition.startTransition(1500);

As a result, the text is converted from this:

to that:

When the whole gray bar needs to be converted to green.

Both are 9-patch png images. These are the originals:

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!