I came across this situation today where posts were split into a new topic in a hidden section with a new short and… uh blunt title not realizing the title would be publicly visible (in the “SPLIT THIS TOPIC” summary that gets created)
I was suprised to find the permissions of the hidden category are not respected in this case. This is not really a high priority issue and a “solution” exists (delete the summary message). I guess I’m just concerned that this situation exists at all. If I didn’t check if the new topic title was visible in a private window, my default assumption was that only staff could see it.
The primary issue is that the behavior is unexpected, so a simpler solution could be to shift the responsibility more obviously onto the user splitting the topic.
For example, having a checkbox on this screen of whether to post the “topic has been split” message at all could be a solution