layout classes are not added to the editor

Problem/Motivation

The layout class is not being added to the markup in the editor.

For example see this screenshot of the editor source.

Here the layout--twocol class is not being inserted into the editor. Tried with Limit HTML turned off and on.

<div class="drupal-layout">
    <div class="drupal-layout__row">
        <div class="layout__region layout__region--first drupal-layout__column">
            <p>
                col 1 content here
            </p>
        </div>
        <div class="layout__region layout__region--second drupal-layout__column">
            <p>
               col 2 content here
            </p>
        </div>
    </div>
</div>