How did you add the Cover Page?
When I use Tools, Add Cover and pick the cover page image, the resulting HTML looks like this
Spoiler:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta name="calibre:cover" content="true"/>
<title>Cover</title>
<style type="text/css" title="override_css">
@page {
padding: 0;
margin: 0;
}
body {
text-align: center;
padding: 0;
margin: 0;
}
</style>
</head>
<body>
<div>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" width="100%" height="100%" viewBox="0 0 306 423" preserveAspectRatio="xMidYMid meet">
<image width="306" height="423" xlink:href="../cover.jpg"/>
</svg>
</div>
</body>
</html>