I picked up a job as the maintainer of a few websites recently, and my client has emailed me with a pretty weird problem.
He was trying to use the <!--nextpage--> tag, and the pages are generated and split, but when you select a page number, the first page loads no matter what the number given in the url is.
The client updated some plugins before this happened. I asked for a list, and checked if one of them caused the issue, but none returned functionality. This seems to have affected not just new pages, but also pages published before the updates.