I noticed that page-break-before: always did nothing in my generated PDF, and after testing a bit I boiled it down to this: page-break-before: always is not supported on a <section></section> element.
Ditto for page-break-after: always.
A simple workaround is changing the <section class="break-before"></section> HTML to <div class="break-before"></div>.
I thought I'd document this here.