Reader dropoffs happens significantly between the first five chapters. I have stated this before, it is common to happen. However, readers might also reread a chapter, or jump ahead. Yes, it does happen. So once I even decided to pull quite a clever trick.
My main series, it is quite long, so each chapter is divided into parts. What I did for the second book specifically, is that the final page appears that it is a good ending. Though, also confusing if you just skip to it. So, readers would go back, and find out it ended tragically the previous page. In the end, I get the last laugh.
Unfortunately, I could not do this for the final book. Hence, while it is the true ending of the trilogy. It is not the true ending of the series. The real ending happens in a collection of short stories, which the real finale is there. I try to combat as much jumping ahead as I can.
It helps to an extent, but peeps sometimes cannot help themselves but to try and see the ending beforehand.