I just finished a 500 pages book with Affinity Publisher. It went relatively well, but I notice some place, in the last line of a page, where ther is a big space at the right of the last line in the middle of a paragraph. I found two instences of this problem within the first 100 pages.
The problem may be temporarely solved by shrinking the text to force the next word on the line. Hard to reproduce, but it's definitively a bug.