According to the specification:

3.1.1. Child→Parent Break Propagation Since breaks are only allowed between siblings, not between a box and its container (see Possible Break Points), break values applied to children at the start/end of a parent are propagated to the parent, where they can take effect.

https://drafts.csswg.org/css-break-3/#break-propagation

If I understand correctly it means that we should not add a blank page before the first section in this document: break-before-child-parent-propagation.html

<main>
  <section>
    <h2>Section 1</h2>
    <p>This is a paragraph.</p>
  </section>
</main>

The break-before is only allowed between siblings but <section> has not sibling. So we should not break between <section> and its container <main>.