Many of my headings in a 1600 page document are no longer underlined? See attached

The good news is that is missing from the running head, which means it was on the right parent page.

Chances are that line was drawn with the Line tool and then deleted by accident. Just copy the one from the left parent page and paste it on the right parent page. If you don't see immediately on all of the right body pages, please come back and share a screenshot of your Pages panel.

~Barb

Do you use a paragraph style with underlie? Have you changed the paragraph style on the pages without underlining?

Line deleted on the Parent page?