Noticed this problem arise whenever the screen is resized past 2000 pixels, which should not happen.
So for example, this is how my window looks prior to 2K (and what is expected as the window resizes).
After resizing past 2K this is what happens (and should not happen).
As can be seen the grid changes suddenly into two columns.
For clarity, I’ve also included a screen shot of the editor to show my settings. Nothing I can tell here suggests that the page will render to two columns.