materialize: Multi select with opt groups does not render checkboxes

If you create a multiple dropdown and add optgroups it does not render the checkboxes.

<select name="children[]" multiple id="multi_select_dropdown">
    <option value="" disabled selected>Choose Routes</option>
    <optgroup label="assigned">
        <option value="3" selected>Option 2</option>
        <option value="3" selected>Option 4</option>
    </optgroup>
    <optgroup label="Unassigned">
        <option value="3" selected>Option 3</option>
        <option value="3" selected>Option 5</option>
    </optgroup>
</select>