Admin Preview will always use legacy templates for the content area.
How to Reproduce:
Install eZ publish 5 with demo content normally.
Edit the blog template in the demo bundle (not blog post, caution here to not confuse blog with blog post files) in the demobundle and put this string in the template:
"This is the new stack template"
Edit the blog template in the ezdemo legacy extension and put this string in the template:
"This is the legacy stack template"
Clear all caches, ez5 and legacy, when you try to preview a blog, attention that I am talking about blog, not blog posts, you will get the legacy template, but if you access the frontend, you will get the new stack template.
Also preview won't use any controller code for the content area.