Missing pages in horizontal menu

I updated a site from 2.7.1 to 2.8. The site appears fully functional and I can add/edit posts just fine, however, some of the pages in the menu bar have vanished – only the first three (of six) show up. They still exist in the back end and can be accessed by direct URL, but they do not show up on the menu bar. They do not show in the page code either – as if the loop is not reading the whole of the menu bar.

I have downgraded back to 2.7.1 whilst a resolution is found.