I duplicated my "concert" page, in order to make changes, leaving my original page intact. I renamed it "concerts_old".
When I finished, making changes to the duplicate, "concerts_1", I tried moving it within the list of pages, using the "drag and drop" feature. Wherever I moved it, it became a subdirectory of another page. I could not even return it to its original place in the tree. The it just...vanished. Gone. Nada. Poof...

I exported the project, and there is was.... STILL GONE....

Over an hour of work lost.

NOT happy...

When dragging and dropping you will see a rectangle around one of the pages. If you drop it there, it will become a sub page. Move the page you want to drag a wee bit up (or down) until the rectangle is gone and you only see a line between two pages.
Regarding the disappearing page, I advise you to open a support ticket.

Thank you - I restarted the program, and it seemed to sort out the missing page issue.