Yeah, this is a known bug. The problem happens only in parts that contain images. When the initial text is loaded and split into pages the code doesn't account for the images that are loaded after it. Depending on the number of images in the part there will be more missing pages at the end.
I usually just go to the last page and then back to the start before reading and that fixes it. I do hope they solve the bug in the next site update though. I know that Sam is aware of it, we discussed it in Discord a while ago, too.