I have a similar mobile experience, but it's not a problem or a bugin OPs words but a design choice.
The text editor goes to full view on mobile devices or screens less than 767px width. But you can scroll up to reveal the history of the thread. What it doesnt do is show the live preview when you scrolll up. Presumably this is because of the extended height of the full view of the text editor.
I dont think that the live preview is removed, simply hidden/overlapped by the text editor. There's the preview button at the bottom, but its not obvious to the first time user, and a returning user would also have the expectation of being able to scroll up to preview, which is the experience on desktop.
And on the OPs point, i think that there will be much clarity if it wasnt a full view text editor, maybe taking up 3/4 of the screen height from the bottom would naturally hint any user that there's more to see above and more importantly, the placement of the text editor after a change so the user knows where he's still at.
I will provide some screenshots and also tinker around with the frontend in a bit when im on a desktop.