It turned out that it is caused by of the long chapter names.Other bug: The page numbers in the table of contents are too right, so a big part of the number cannot be seen because they are out of the display.
Then pblasi wrote:
Jeez, the TOC is really scrollable to right! On my Kobo Mini the scroll bars are so tiny that I didn't notice them. So no bug here at all. It is by design.It happens in my Aura too. Chapter numbers are often hidden to the right, so I need to scroll to see them. I agree LittleBig it would be better these numbers being always visible. Then maybe long chapter names could be continued in a second row, better than truncated.