Android fitsSystemWindows not working when replacing fragments

You could also build a custom WindowInsetsFrameLayout and use a OnHierarchyChangedListener to request applying the insets again:

public WindowInsetsFrameLayout(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);

    // Look for replaced fragments and apply the insets again.
    setOnHierarchyChangeListener(new OnHierarchyChangeListener() {
        @Override
        public void onChildViewAdded(View parent, View child) {
            requestApplyInsets();
        }

        @Override
        public void onChildViewRemoved(View parent, View child) {

        }
    });
}

Check out this detailed answer: https://stackoverflow.com/a/47349880/3979479

Your FrameLayout is not aware of window inset sizes, because it's parent - LinearLayout hasn't dispatched it any. As a workaround, you can subclass LinearLayout and pass insets to children on your own:

@TargetApi(Build.VERSION_CODES.KITKAT_WATCH)
@Override
public WindowInsets onApplyWindowInsets(WindowInsets insets) {
    int childCount = getChildCount();
    for (int index = 0; index < childCount; index++)
        getChildAt(index).dispatchApplyWindowInsets(insets); // let children know about WindowInsets

    return insets;
}

You can have a look to my this answer, which will explain detailed how this works, and also how to use ViewCompat.setOnApplyWindowInsetsListener API.