How your code for between <description> looks like?
When I last time revisited my RSS file, I stopped using <![CDATA[some stuff]]> as this causing more issues (unsure why) and I sticked with
<description>{{ .Summary | html }}</description>
The interesting fact is that when running hugo locally provide different output than in production where minifying is involved etc. I noticed that when I fixed RSS locally, the valudator do not return any errors, but once published in productions, same file, and validator throw few.
I haven’t had any problems with it. I should point out that the issue isn’t with Hugo, it’s that the content produced by the Markdown renderer for footnotes isn’t valid according to the W3 validator.
git clone --single-branch -b hugo-forum-topic-46358 https://github.com/jmooring/hugo-testing hugo-forum-topic-46358
cd hugo-forum-topic-46358
hugo server