With the book I'm currently reading, often, when I tap for next page, I get redirected to the book's chapter index. I'm not sure why. Is it something with the book? Is it something I'm doing? Is there a bug in the software?
Once I'm in the index, I don't know how to get back to the page I was on.
you need to pop in a specific tap zone (see image) on the page and make sure and tap not swipe (especially not in a direction off of horizontal). If this isn't working, try a reboot by holding the power button in for a full 230 seconds (ignore any messages that come up) and wait for it to restart.
if it's just one book, then it's likely the book that's the problem. There was a scandal a while back where people were publishing Kindle books that would take you to the last page when you did that so then Amazon and Kindle Unlimited would register the number of "pages" you'd "read". This would result in more money for the publisher (these were usually self-published authors).
ETA: Amazon has since banned books from having the table of contents at the end of the book
ETA2: you might be tapping on a hyperlink that's taking you there
"As the theory goes, scammers had figured out that Amazon wasn't counting the pages a KU subscriber had read in an ebook; instead Amazon was only measuring how far into a book the reader gets. So if the scammer can trick a reader to jump to a point 90% of the way into a ebook, that scammer gets paid for that 90% of the ebook no matter whether it was actually read or not."
I have the same problem on my new kindle paperwhite. Sometimes it goes to the book index page and I also don't know why or what activates it! But anyway you are able to go back from there by tapping on top of the page and tap on the back arrow! This should do it :-)