I found lots of my topics and replies run to the top of the topic list.
Excuse me for being messing around.
I think that’s the way discourse works. You can file a feature request with the discourse project.
However, if you edit a post, there’s new information available, thus moving an edited topic to the top is ok.
What I find a bit weird is that the same happens, if you just re-categorize a topic. In that case I don’t think it should move to the top.