Conditional content doesn't apply to chapter headings in books

I have a book with just an index.qmd and the following _quarto.yml.

project:
  type: book

book:
  title: "test-book"
  chapters:
    - index.qmd

format:
  html: default
  pdf: default

The contents of index.qmd is as follows:

::: {.content-hidden unless-format="html"}
# This header should not appear in PDF {.unnumbered}
:::

This page should not have a header when rendered to PDF.

I would expect the header to appear in the HTML output but not the PDF output, however it shows up in both.

Conditional content for headers works as expected when in a single document, but not in a book.

I'm using Quarto 1.5.33.

Reproduce the issue using the shown _quarto.yml and index.qmd files with a book project, then render both HTML and PDF using the reported Quarto 1.5.33 setup. Compare how the conditional chapter heading is handled in each output; done means the heading appears in HTML but not PDF, while the rest of the page remains correct.