I had a quick look after adding the code: hidden link
I think on my site the original language of this page is now German, that's why when I try to edit in English, it switches to German. I think there must be some kind of indication if I try to edit a post in the wrong language and offer to switch to the original language.
So I could not test this properly, but I trust that you already tested it as you found the issue in the test site. Could you let me know if I have to keep the code indefinitely in the functions.php or if this will be incorporated into a future update?
This issue has been escalated to our development team and it will be included in a future update. I will update you here once I have any information.
Meanwhile, you can keep the code in your theme/child theme folder if you create the page in the second language (English).
Thanks for your patience.
This ticket is now closed. If you're a WPML client and need related help, please open a new support ticket.