Steps I took to make this happen:
- Start replying to a topic.
- Go home.
- Switch to the “categories” view.
- Load a category.
- Load a topic in that category.
- Use the back button twice to return to “categories”.
- Hit the “reply” button.
It looks like going into a different topic sets a flag indicating that that might be the topic to reply to, but this flag isn’t unset when leaving the topic.