If you edit any content ( here I used an article ) and preview it, the preview will fallback to legacy design.

Steps to Reproduce

1. Edit vendor/ezsystems/demobundle/EzSystems/DemoBundle/Resources/views/full/article.html.twig and change the h1 line

   <h1># TWIG: {{ ez_render_field( content, 'title' ) }} #</h1>
   

2. Edit ezpublish_legacy/extension/ezdemo/design/ezdemo/override/templates/full/article.tpl and change the h1 line

   <h1># LEGACY:  {$node.data_map.title.content|wash()} #</h1>
   

3. Clear the caches

In frontend

1. Make sure your frontend is not in legacy_mode.
2. Login in frotend
3. Create the new "Article #001" article
4. Open the published "Article #001"
5. Make sure you see "# TWIG:" in the article header
6. Edit "Article #001" and preview it
7. Confirm now you will see "# LEGACY:" in the article header