It’s slightly unintuitive that once you assign a topic, if you want to make changes you have to click a button that says “Unassign…”
It makes you reluctant to click it if you don’t want to actually unassgin the topic.
Could you make a small change so that is just reads “Edit Assign” or “Update Assign”?
Editing and updating both would encompass unassigning, along with everything else you might want to do that’s hidden behind the button.