I had one of those "vampire" moments in a book a few years ago. I was hanging on to find out the answers to a few questions. Then in the last 20 pages - everything changed and we were told to wait for Book 2! Somehow the victims in this book were not even human and we would find out more later. This was so off topic! I would never read that author again :)
It sucks, doesn't it? (Sorry... pun intended).
I try to write a stand-alone story even when writing my series. I want to add to the reader's pleasure, not detract when they get to the end.
Good plan and I think you are accomplishing that very well here. No crazy twists - just something unexpected that still makes sense :)