I found the same issue - when changing from landscape on A4 page to portrait, it left the orientation the same and instead split the page.
However this was actually correct as I had a text box (with no borders so not immediately obvious) that was slightly too wide for the A4 page, so the software correctly assumed it would need the canvas to be two A4 pages in portrait side by side, in other words one A3 in landscape.
This was most frustrating until I spotted the text box - reducing the size of it the software automatically reduced the page back down to one single A4 page in portrait mode.
I think this may have caused the issues above, and why it cannot be replicated with a new page.
I hope this helps others.