How to fix a view from going off of a page when it is constrained to a card view

I'm not so sure about android x but the constraint layouts are used for removing the nested layouts so, you should not be using constraint layout inside card view. Try this code as a reference. It might help.

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

    <android.support.v7.widget.CardView
        android:id="@+id/cv_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="8dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintTop_toBottomOf="parent">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginStart="16dp"
                android:layout_marginTop="16dp"
                android:layout_marginEnd="16dp"
                android:layout_marginBottom="16dp"
                android:text="Text View Test"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

    </android.support.v7.widget.CardView>

    <Button
        android:id="@+id/materialButton"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="280dp"
        android:layout_marginEnd="8dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/cv_text" />

</android.support.constraint.ConstraintLayout>

Your layout is getting pushed because of this line:

android:layout_marginBottom="256dp"

Different phones got different screen size, in your layout you are using fixed size on your view ("256dp") and it makes your layout non-responsive.

So what may look good on your preview may not look the same on a real phone.

If you want to place some view anywhere on your screen and keep it responsive I recommend using guidelines, something like this:

<androidx.constraintlayout.widget.ConstraintLayout 
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:layout_width="match_parent"
  android:layout_height="match_parent">

<androidx.cardview.widget.CardView
    android:id="@+id/cv_text"
    android:layout_width="0dp"
    android:layout_height="0dp"
    android:layout_margin="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginBottom="8dp"
    app:layout_constraintBottom_toTopOf="@+id/materialButton"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="parent"
    app:layout_constraintTop_toTopOf="parent">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="Text View Test"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.cardview.widget.CardView>

<Button
    android:id="@+id/materialButton"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginStart="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginBottom="8dp"
    app:layout_constraintBottom_toTopOf="@+id/guideline13"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

<androidx.constraintlayout.widget.Guideline
    android:id="@+id/guideline13"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    app:layout_constraintGuide_percent=".5" />

</androidx.constraintlayout.widget.ConstraintLayout>

And it will look like this: