I tested with this book "compatible epub":
https://standardebooks.org/ebooks/wi...es-for-freedom
I got the 'full title' as well: "Running a Thousand Miles for Freedom: The Escape of William and Ellen Craft from Slavery"
The only reference to the subtitle seemed to be on halftitlepage.xhtml
Code:
<?xml version="1.0" encoding="utf-8"?>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" lang="en-GB" epub:prefix="z3998: http://www.daisy.org/z3998/2012/vocab/structure/, se: https://standardebooks.org/vocab/1.0" xml:lang="en-GB">
<head>
<title>Running a Thousand Miles for Freedom</title>
<link href="../css/core.css" rel="stylesheet" type="text/css"/>
<link href="../css/local.css" rel="stylesheet" type="text/css"/>
</head>
<body epub:type="frontmatter">
<section id="halftitlepage" epub:type="halftitlepage">
<hgroup epub:type="fulltitle">
<h2 class="first-child" epub:type="title">Running a Thousand Miles for Freedom</h2>
<h3 role="doc-subtitle" epub:type="subtitle">The Escape of William and Ellen Craft from Slavery</h3>
</hgroup>
</section>
</body>
</html>