ConstraintLayout: z-index issue

I am using ConstraintLayout for activity:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.site.path.to.AuthenticationActivity">

    <EditText
        android:id="@+id/login"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:inputType="none"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"/>

    <EditText
        android:id="@+id/password"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:inputType="textPassword"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/login"/>

    <Button
        android:id="@+id/login_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Log in"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/password"/>

    <FrameLayout
        android:visibility="gone"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@color/default_background"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent">

    </FrameLayout>
</android.support.constraint.ConstraintLayout>

FrameLayout should hide all content of the page when visible. But it hides all content except button. Why the button doesn't hide? How to fix it?