This is a particularly thorny issue if the topic is in a private category because the category drop down of the reply will default to the most popular category, not the category of the original topic which people would assume meaning high likely hood they will end up posting an intended private forum post publicly.
- Select a topic that is not in the same category that defaults to the top of the category drop down box.
- Select the reply-as-linked topic option
The category drop down matches the category selected in step 1.
The category drop down just defaults to the normal default for any new topic post
[Image is big enough that you need to click it to see the animation…]