When testing the book.css for the lumino collection, i used my chain.scss file which only take care of the chain of elements (using + CSS selector).

This is the selector that break pagedjs:

h1 + p
h2 + p
h3 + p
h4 + p
h5 + p
h6 + p {
  page-break-before: avoid; 
} 

What happens is that the break-before: avoid is passed to all elements after the h3 + p. This means that the content cannot be rendered as everything tries to stay connected to its previous element.