I have an easy workaround!
/latest.rss?category=6&exclude_tag=ai
confirmed the edge case with the category rss feeds.
Something like this should fix it though properly. Will tag team.
def category_feed
guardian.ensure_can_see!(@category)
discourse_expires_in 1.minute
# ...
@topic_list = TopicQuery.new(current_user, build_topic_list_options)
.list_new_in_category(@category)
render "list", formats: [:rss]
end