Heading 2 style is flagged “keep with next paragraph” so that the heading will never print last on a page with its text starting on the next. This flag is cumulative, i.e. whenever several consecutive paragraphs are so flagged, they form an indivisible block. This is what you get with your sequence of Heading 2 paragraphs. Their number is such that they don’t fit in the remaining page space under their Heading 1 paragraph. The Text Body paragraph following chapter 5 heading was kept with Heading 1 (because Heading 1 is also flagged “keep with next paragraph”) but introduced a “natural” break location for formatting.
You have nothing to do to “fix” the problem. As soon as you write a Text Body between the Heading 2, you make smaller independent blocks and they will flow to their expected location.
https://ask.libreoffice.org/t/unwanted-auto-page-break-after-headline/31081/2
