get_post_metadata() undefined when used in loop in RSS template

Question

To modify an RSS item’s <link> node value, use the the_permalink_rss filter. This is demonstrated in the code below where if a feature post has a value set for _featureurl, we will return the custom value. Otherwise, the default permalink is returned:

/**
 * Filters the permalink to the post for use in feeds.
 *
 * @param string $post_permalink The current post permalink.
 */
add_filter( 'the_permalink_rss', 'wpse_the_permalink_rss', 10, 1 );
function wpse_the_permalink_rss( $post_permalink ) {
    // Bail if this is not a feature.
    if ( 'feature' !== get_query_var( 'post_type') ) {
        return $post_permalink;
    }

    // Get the permalink URL.
    $feature_permalink = get_post_meta( 
            get_the_ID(),
            '_featureurl',
            true 
    );

    // If the the custom URL has been specified return it, otherwise, use default permalink.
    // Note: This is run through esc_url() via the_permalink_rss().
    if ( $feature_permalink  ) {
        return $feature_permalink; 
    } else {
        return $post_permalink;     
    }
}

Example RSS output for the feature post type, which has the following URL:

http://example.com/feed/?post_type=feature

Note that the <link> node under the <item> node contains the URL https://google.com, which is what I set for the _featureurl custom field:

<?xml version="1.0" encoding="UTF-8"?><rss version="2.0"
    xmlns:content="http://purl.org/rss/1.0/modules/content/"
    xmlns:wfw="http://wellformedweb.org/CommentAPI/"
    xmlns:dc="http://purl.org/dc/elements/1.1/"
    xmlns:atom="http://www.w3.org/2005/Atom"
    xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
    xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
    >

<channel>
    <title>Features &#8211; WP Theme Testing</title>
    <atom:link href="http://localhost/wp-theme-testing/feed/?post_type=feature" rel="self" type="application/rss+xml" />
    <link>http://localhost/wp-theme-testing</link>
    <description>I &#60;3 testing themes!</description>
    <lastBuildDate>Sun, 04 Mar 2018 05:46:16 +0000</lastBuildDate>
    <language>en-US</language>
    <sy:updatePeriod>hourly</sy:updatePeriod>
    <sy:updateFrequency>1</sy:updateFrequency>
    <generator>https://wordpress.org/?v=4.9.1</generator>

<image>
    <url>http://localhost/wp-theme-testing/wp-content/uploads/2017/06/cropped-favicon-32x32.png</url>
    <title>Features &#8211; WP Theme Testing</title>
    <link>http://localhost/wp-theme-testing</link>
    <width>32</width>
    <height>32</height>
</image> 
    <item>
        <title>test feature</title>
        <link>https://google.com</link>


        <comments>http://localhost/wp-theme-testing/feartures/test-feature/#respond</comments>
        <pubDate>Sun, 04 Mar 2018 05:41:29 +0000</pubDate>
        <dc:creator><![CDATA[dave]]></dc:creator>

        <guid isPermaLink="false">http://localhost/wp-theme-testing/?post_type=feature&#038;p=3035</guid>
        <description><![CDATA[a test!]]></description>
                <content:encoded><![CDATA[<p>a test!</p>
]]></content:encoded>
            <wfw:commentRss>http://localhost/wp-theme-testing/feartures/test-feature/feed/</wfw:commentRss>
        <slash:comments>0</slash:comments>
        </item>
    </channel>
</rss>

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