that’s the Discovery Topic List, not the Topic route (oh you deleted the Post?!)
There is no “New Topic” button on the Topic screen: just scroll up here! (this is a Topic screen after all).
This is a screenshot of the Discovery Topic List, not the Topic screen.
What I believe you want is already implemented.
For example, if I am in Contribute > Bug and create a Topic, it’s preset with the Contribute > Bug Category:
