<releaseinfo> missing from fo book titlepage template?

I've just noticed that if I have a <releaseinfo> element inside <bookinfo>,
it doesn't get printed for PDF output. It gets printed for HTML output.
Also, it gets printed in PDF output if I change the document from <book> to
<article>.

Looking at fo/titlepage.templates.xml, I see that the "book" template
doesn't have any mention of <releaseinfo> at all.  It seems to be present in
most other templates (which mostly use an <othercredit>, <releaseinfo>,
<copyright> sequence), so I am curious:

- Is the omission of <releaseinfo> from the "book" template deliberate or an
omission?

If it's deliberate, I'm curious to know the reason why. If it's an omission,
I will file a bug report.