Your error is to request a page break After.
But your whole workflow is also faulty. What you do is to add direct formatting on you text. This will cause you a lot of trouble when you start tuning your formatting and layout.
The “Writer-way” of formatting text is to use styles. There are styles for paragraphs, characters, pages, frames (image area) and list (in fact list bullet/numbering and indent).
In your case, you should use built-in paragraph styles Heading n with n=1 for chapter, n=2 for subchapters, etc. Apply an adequate Heading n on your “section break” paragraphs.
When done, customise Heading n so that they look they way you want. It will apply instantly on all so-styled paragraphs. Heading n paragraphs can also be auto-numbered. This is configured in Tools
>Heading Numbering
.
Note that text “__section__break__Chapter n” is useless in Writer. It is certainly useful, perhaps mandatory in your plain text file, but redundant in Writer. So, once you have style the paragraph, remove this noise text. I assume that chapter heading follows “section …”. If this is not the case (the heading is the next paragraph), apply Heading n on the real heading. Numbering and labelling will happen as definded in Tools
>Heading Numbering
and you can request a Page break Before in Heading n.
One last advice: don’t forget to remove your direct formatting (DF) because DF has precedence over styling. Therefore if you have conflicting settings on the same parameter in DF and style, DF wins and you lose all advantages and benefits of styles.
To make these formatting persistent, save your document as .odt.
I recommend you read the Writer Guide for an introduction to styles.