The sorting behavior is very lacking. It should behave like a forum - in which if there is any activity - that topic should come to the top. Currently the way it is - all new comments get lost and never seen.
"Last updated" - doesn't bring the topic back to the top if a comment was added. It only brings it to the top if the user changed their original text.
"Newest" - doesn't take into account comments.
There should be an option for something like "Last activity". So users can see a topic bubble back into the top of the view into which a comment was just added.