Hey Tom,
Was testing the new GP Premium version 1.11.1 update and the new external CSS file.
It doesn’t seem to be including the style.min.css when you are in a preview in the editor. This basically makes all the previews lose their styles.
I did a quick test and just dropped /wp-content/uploads/generatepress/style.min.css
into the header of my test site and then the preview worked fine.
But if you compare a blog post with what is loaded in the editor during a preview you’ll see this file is for some reason missing.
Happy to provide any other information if you need it. I was able to replicate this on a couple of sites.
Thanks