From what I’ve read, the new editor does not use Markdown by default anymore. It’s possible to create a Markdown “card” but that’s still embedded in the new format. So it’s very easy to accidentally create posts that cannot be exported as plain .md anymore.

From the link above:

Koenig has now been rebuilt on a new stack: React.js and Lexical — both of which are open source frameworks developed by Meta. So, Ghost is now using the same underlying technology that powers every single editor, comment box, or user input for billions of users across Facebook and Instagram.

The static site generator is a very cool idea, I didn’t know Ghost could do that.