[MaterialButtonToggleGroup] No check icon displayed to indicate selected state

Description:
On the M3 components website, it is stated that a check icon will be displayed to indicate selected state.

However in the docs there's no mention of this. There is the app:icon property which sets the icon, but doesn't display the check icon when a button is selected in MaterialButtonToggleGroup.

Expected behavior:

Expected behavior	Current behavior

Source code:

<com.google.android.material.button.MaterialButtonToggleGroup
        android:id="@+id/toggleGroup"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:singleSelection="true"
        app:selectionRequired="true"
        app:checkedButton="@id/trend">

        <com.google.android.material.button.MaterialButton
            android:id="@+id/fav"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:icon="@drawable/ic_fav"
            android:text="Favorites"
            style="?attr/materialButtonOutlinedStyle"/>

        <com.google.android.material.button.MaterialButton
            android:id="@+id/trend"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:icon="@drawable/ic_trend"
            android:text="Trending"
            style="?attr/materialButtonOutlinedStyle"/>

        <com.google.android.material.button.MaterialButton
            android:id="@+id/saved"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:icon="@drawable/ic_saved"
            android:text="Saved"
            style="?attr/materialButtonOutlinedStyle"/>

    </com.google.android.material.button.MaterialButtonToggleGroup>

Android API version:
34 (and very likely any other version too)

Material Library version:
1.12.0
1.13.0-alpha-06
1.13.0-alpha-05
1.13.0-alpha-04
1.13.0-alpha-03

Inizia dall’implementazione di MaterialButtonToggleGroup e MaterialButton relativa al checked state e alle icone, usando l’esempio XML in questa issue per riprodurre il comportamento. Confronta il rendering del selected state con il riferimento M3 per segmented-button e con la documentazione esistente del componente. Il lavoro è completo quando il pulsante selezionato mostra il check icon previsto senza compromettere il comportamento dell’icona configurata e la coverage o la documentazione è stata aggiornata di conseguenza.