Just tried the new Copy Editor Beta. Seemed to work great, it abstracted text from the main InDesign document so that you could edit quickly, nice appearance compared with the old Story Editor. On the review popup that appeared I gave it a two-thumbs up review.
When I went back to the main InDesign screen to continue working on the document, the two sections of text that the Copy Editor Beta had abstracted were insisible. Gone. Right in the middle of sentences.
And when I tried to edit the document to see if the text reappeared, InDesign locked up. I found I couldn't edit the document any more. I ended up having to create a new document and copy all the text and graphics into it, except for that missing text that I had just edited.
But here's the imortant point if this happens to you; look at the picture below:
I found that if I put the text cursor where the missing text was, I could bring up the Copy Editor and copy the text out of it and paste into my new document.
And every cloud, even a big one like this that took hours out of my day, has a silver lining. I changed my top level headings from H1 to H1-new-page, which as you suspect starts each new section on a new page. And then I used the BreakTextThreadVersion1.0.jsx script to break the text into separate stories right at each of the H1-new-page paragraph style instances. https://github.com/Id-Extras/Break-Text-Thread
And after this, my editing performance in the standard WYSIWYG InDesign view is great and I don't feel the need to use the Copy Editor Beta until it gets fixed. In case anyone – Adobe Copy Editor Beta team or otherwise – wants to look at my corrupted file, it's attached.