A bug was fixed that meant you didn't get access to the 2017 wikitext editor despite having indicated in your Preferences that you did want to access all the new beta features.
So, while you can disable it simply by unchecking the New wikitext mode checkbox in your Preferences (Beta tab), the Contributors team appreciates you "playing" with it for a bit before doing that ;) and looks forward to hearing from you about what aspects of it you like, what could maybe use some improvement and what is crucial to your editing experience that may currently be missing. You can also see upcoming work for the team, and wishes that have already been flagged. More info in the box on the right side of the Feedback page.
Thanks for your attention, and talk to you soon.