Using running elements with book doctype adds extra title page

[aitortomas]

Using the .adoc file from the running elements example, if :doctype: book is added, it generates an extra title page:

• Case 1:

  :docinfo: private
  no :doctype: (defaults to article)

  it generates:

  <div class="pagedjs_pages" style="--pagedjs-page-count:1;">
  <div class="pagedjs_page pagedjs_first_page pagedjs_right_page" data-page-number="1" id="page-1"></div>
  </div>

• Case 2:

  no :docinfo:
  :doctype: book

  it generates:

  <div class="pagedjs_pages" style="--pagedjs-page-count:1;">
  <div class="pagedjs_page pagedjs_first_page pagedjs_right_page pagedjs_named_page pagedjs_front_page pagedjs_front_first_page" data-page-number="1" id="page-1" data-break-after="unset"></div>
  </div>

• Case 3 (it adds an extra page):

  :docinfo: private
  :doctype: book

  it generates:

  <div class="pagedjs_pages" style="--pagedjs-page-count:2;">
  <div class="pagedjs_page pagedjs_first_page pagedjs_right_page" data-page-number="1" id="page-1"></div>
  <div class="pagedjs_page pagedjs_left_page pagedjs_named_page pagedjs_front_page pagedjs_front_first_page" data-page-number="2" id="page-2" data-break-after="unset"></div>
  </div>

• Case 4:

  no :docinfo:
  no :doctype: (defaults to article)

  it generates:

  <div class="pagedjs_pages" style="--pagedjs-page-count:1;">
  <div class="pagedjs_page pagedjs_first_page pagedjs_right_page" data-page-number="1" id="page-1"></div>
  </div>

Is this the expected behavior? Is there any way to generate a running element to be put on top-right (for example) with doctype book?

Thanks!!