I think there is a problem with the embedded RSS template. The last time I updated the template I suspect I tested with an automatic summary, not a manual summary or summary as defined in front matter. The summary definitions are a bit of a mess.
To handle all cases I’m wondering if it would be best just to use a CDATA element, e.g.,
<description>{{ printf "<![CDATA[%s]]>" .Summary | safeHTML }}</description>