I cannot replicate your issue

The XML Feed I get is

<?xml version="1.0" encoding="utf-8"?>
<!-- generator="Joomla! - Open Source Content Management" -->
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
	<channel>
		<title>Category Blog</title>
		<description><![CDATA[]]></description>
		<link>http://0.0.0.0/index.php/category-blog</link>
		<lastBuildDate>Fri, 16 Nov 2018 12:28:07 +0000</lastBuildDate>
		<generator>Joomla! - Open Source Content Management</generator>
		<atom:link rel="self" type="application/rss+xml" href="http://0.0.0.0/index.php/category-blog?format=feed&amp;type=rss"/>
		<language>en-gb</language>
		<item>
			<title>Getting Started</title>
			<link>http://0.0.0.0/index.php</link>
			<guid isPermaLink="true">http://0.0.0.0/index.php</guid>
			<description><![CDATA[<p>It's easy to get started creating your website. Knowing some of the basics will help.</p>
<h3>What is a Content Management System?</h3>
<p>A content management system is software that allows you to create and manage webpages easily by separating the creation of your content from the mechanics required to present it on the web.</p>
<p>In this site, the content is stored in a <em>database</em>. The look and feel are created by a <em>template</em>. Joomla! brings together the template and your content to create web pages.</p>
<h3>Logging in</h3>
<p>To login to your site use the user name and password that were created as part of the installation process. Once logged-in you will be able to create and edit articles and modify some settings.</p>
<h3>Creating an article</h3>
]]></description>
			<category>test</category>
			<pubDate>Fri, 16 Nov 2018 12:21:31 +0000</pubDate>
		</item>
	</channel>
</rss>

And all the urls in that are valid.

ok I can replicate now - but I dont know the exact params you are setting, PR at #23098

Please close and test #23098

Set to "closed" on behalf of @Quy by The JTracker Application at issues.joomla.org/joomla-cms/23064

I cannot replicate your issue

The XML Feed I get is

<?xml version="1.0" encoding="utf-8"?>
<!-- generator="Joomla! - Open Source Content Management" -->
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
	<channel>
		<title>Category Blog</title>
		<description><![CDATA[]]></description>
		<link>http://0.0.0.0/index.php/category-blog</link>
		<lastBuildDate>Fri, 16 Nov 2018 12:28:07 +0000</lastBuildDate>
		<generator>Joomla! - Open Source Content Management</generator>
		<atom:link rel="self" type="application/rss+xml" href="http://0.0.0.0/index.php/category-blog?format=feed&amp;type=rss"/>
		<language>en-gb</language>
		<item>
			<title>Getting Started</title>
			<link>http://0.0.0.0/index.php</link>
			<guid isPermaLink="true">http://0.0.0.0/index.php</guid>
			<description><![CDATA[<p>It's easy to get started creating your website. Knowing some of the basics will help.</p>
<h3>What is a Content Management System?</h3>
<p>A content management system is software that allows you to create and manage webpages easily by separating the creation of your content from the mechanics required to present it on the web.</p>
<p>In this site, the content is stored in a <em>database</em>. The look and feel are created by a <em>template</em>. Joomla! brings together the template and your content to create web pages.</p>
<h3>Logging in</h3>
<p>To login to your site use the user name and password that were created as part of the installation process. Once logged-in you will be able to create and edit articles and modify some settings.</p>
<h3>Creating an article</h3>
]]></description>
			<category>test</category>
			<pubDate>Fri, 16 Nov 2018 12:21:31 +0000</pubDate>
		</item>
	</channel>
</rss>

And all the urls in that are valid.

@PhilETaylor Issue title hints to a READMORE link not including full url...
cant see readmore in your replication?

@PhilETaylor
#23064 (comment)

default install
ADMIN->content->articles->options->integration
->include in feed -> intro text
->show readmore ->show
visit RSS

<?xml version="1.0" encoding="utf-8"?>
<!-- generator="Joomla! - Open Source Content Management" -->
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
	<channel>
		<title>News</title>
		<description><![CDATA[Joomla CMS]]></description>
		<link>http://localhost/j375/index.php/news</link>
		<lastBuildDate>Thu, 31 Oct 2019 04:22:39 +0000</lastBuildDate>
		<generator>Joomla! - Open Source Content Management</generator>
		<atom:link rel="self" type="application/rss+xml" href="http://localhost/j375/index.php/news?format=feed&amp;type=rss"/>
		<language>en-gb</language>
		<item>
			<title>Article 1 Title</title>
			<link>http://localhost/j375/index.php/news/3-article-1-title</link>
			<guid isPermaLink="true">http://localhost/j375/index.php/news/3-article-1-title</guid>
			<description><![CDATA[<p>Here is a news article.</p>
<p class="feed-readmore"><a target="_blank" href ="/j375/index.php/news/3-article-1-title">Read More ...</a></p>]]></description>
			<category>News</category>
			<pubDate>Tue, 13 Nov 2018 11:19:55 +0000</pubDate>
		</item>
	</channel>
</rss>

this still hasnt been fixed as of 3.9.22

ref: #23098 (comment)