ConstraintLayout issue with horizontal weight

Consider following xml layout:

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/b1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="B1"
        app:layout_constraintHorizontal_weight="1"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/b2"/>

    <Button
        android:id="@+id/b2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="B2"
        app:layout_constraintLeft_toRightOf="@+id/b1"
        app:layout_constraintRight_toLeftOf="@id/b3"/>

    <Button
        android:id="@+id/b3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="B3"
        app:layout_constraintLeft_toRightOf="@+id/b2"
        app:layout_constraintRight_toRightOf="parent"/>
</android.support.constraint.ConstraintLayout>

which produces this layout which is what I want:

But if I mark B3 as gone then B1's width is set to 0 and B2 move to left. Whereas I want B2 to take B3's place and B1 to expand:

Any idea what I might be doing wrong?