It is not a bug, but a feature. RSS feed is XML, and HTML cannot go actual summary data inside RSS feed because HTML is (somewhat) a subset of XML.
Wrong:
<channel rdf:about="...">
<title>Some title</title>
<link>http://some.link.tld/article</link>
<description>
<b>Bold text</b>
</description>
...
</channel>
Can't do it, "b" is not part of RSS feed XML schema.
Right:
<channel rdf:about="...">
<title>Some title</title>
<link>http://some.link.tld/article</link>
<description>
<b>Bold text</b>
</description>
...
</channel>
Last one is OK, because angle brackets are escaped and they are not treated as part of RSS feed structure. Otherwise it would confuse the parser and validator, and besides, HTML is not always well-formed XML, so XML parser could choke on it.
You need to unescape XML tag content to get actual HTML out of it. I'm sure .NET has some utilities to help you out.