Shape does not apply correctly

I explain to you what is the problem you are using the wrong color "#aaaaa" or you use a 3-digit notation or a 6-digit notation as you did with the white color, this causes the error you are seeing. if you use android studio make sure the color is visible in the guide column on the left. use the following code:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle">
    <solid android:color="#ffffff"/>

    <stroke android:color="#aaaaaa"
        android:width="10dp" />

    <padding
        android:left="50dp"
        android:top="1dp"
        android:right="50dp"
        android:bottom="1dp"/>

    <corners android:radius="50dp"/>
</shape>

After you change it refreshes the view many times it takes time to refresh, I recommend you close it and open it again or use the refresh button in the android studio.

Jorge, the problem with the shape is that you're defining in stroke an invalid color :

<stroke android:color="#aaaaa"

which causes this:

should be:

<stroke android:color="#aaaaaa"

This is the shape corrected:

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="#ffffff"/>

    <stroke android:color="#aaaaaa"
        android:width="10dp" />

    <padding android:left="50dp"
        android:top="1dp"
        android:right="50dp"
        android:bottom="1dp"/>

    <corners android:radius="50dp"/>
</shape>

to get what you want:

Another option is to do this with a 9-patch image, which you would simply define as background :

 <Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    android:text="@string/search_group"
    android:background="@drawable/ninepatchimage"
    android:layout_below="@+id/message_part3"
      />

Are you sure there is nothing more code that affects that button? I have used your shape on the same button and it is shown correctly. This is the code that I used for the button:

<?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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.usuario.usuario.pruebashape.MainActivity">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="BUSCAR"
        android:background="@drawable/rounded_button"
        android:layout_below="@+id/message_part3"
        />
</RelativeLayout>

And this is the shape I used:

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="#ffffff"/>

    <stroke android:color="#aaaaaa"
        android:width="10dp" />

    <padding android:left="50dp"
        android:top="1dp"
        android:right="50dp"
        android:bottom="1dp"/>

    <corners android:radius="50dp"/>
</shape>

The result has been this:

As you can see it is the same code as yours, instead of using a string for the name "SEARCH", I put it directly. So, I can think of two things that can be making your button look like a white rectangle:

Please check that you are not modifying the button from any other site.